A New Lead Connecting the Heretic Marcus Directly With St. Mark of Egypt

Here is the Wikipedia entry - notice that no effort is made to explain why Clement employed 'Marcosian' source material:

His [Marcus] doctrines are almost exclusively known to us through a long polemic (i. 13–21) in Adversus Haereses, in which Irenaeus gives an account of his teaching and his school. Clement of Alexandria clearly knew of Marcus and actually used his number system (Stromata, VI, xvi), though without acknowledgement.

Nevertheless I found what followed in the article a particularly useful lead connecting 'Marcus' back to Egypt:

Sulpicius Severus, and others who give the history of the origin of Priscillianism, tell that one Marcus of Memphis brought the Gnostic doctrines into Spain, from whom Agape and Elpidius learned them. Jerome certainly identified this Marcus with the subject of the present article, his notion bring that Marcosian doctrine, which we know from Irenaeus to have been prevalent in Southern Gaul, naturally passed on to the adjacent province of Spain. It is not quite clear whether Jerome felt the chronological difficulties of his theory, which, however, could be easily got over by supposing that the first Priscillianists were to be regarded as having learned from Marcus, not because they had been taught by himself personally, but because they had learned from men who revered him as the founder of their sect.

I did some digging and found Jerome's polemic against Priscillian in his Letter to Ctesiphon (133, 4) by Alberto Ferreuro Revue des Études Augustiniennes, 39 (1993), 309-332 which says that:

According to Sulpicius, a Gnostic Marcus of Memphis was the first to introduce Gnosticism into the Iberian Peninsula, and Agape and Elpidius were his first pupils. They, in turn,were the teachers of Priscillian. Another unique aspect about them is that neither are found in any other heretical lists. Of Agape and Elpidius we know nothing else, but of Marcus there is plenty in the patristic sources, and Jerome had already identified Marcus in his Commentaries on Isaiah (17. 64. 4-5) as the Gnostic heretic 'behind' Agape.

In a letter to Theodora, Jerome commented more about Marcus, citing Irenaeus as his major source. He erroneously called Marcus a disciple of Basilides. In the remainder of his exposition Jerome was consistent with the previous commentaries on Marcus. Jerome accused Marcus of misleading unlearned men and high-born women, and of engaging in unlawful intercourse. The Constitutions of the Holy Apostles called Marcus a spiritual successor of Simon Magus and Hippolytus reports that Marcus even allowed women to offer up the Eucharist.

For Jerome's purpose Marcus confirms the illicit sexual behavior of heretics, the sexually loose women heretics tend to attract, the seduction of weak-minded uneducated men, and lastly but no less important, the unbroken succession with Simon Magus. The reference to Marcus is not without its problems as Virginia Burrus alerts us. Jerome links Priscillian with the Marcus identified by Irenaeus in several places. Sulpicius made a similar connection identifying Marcus as the originator of Gnosticism in the Iberian Peninsula, yet he does not say that Marcus taught Priscillian directly. Recall that Agape and Elpidius were taught by Marcus according to Sulpicius. Some scholars have argued back and forth on thequestion whether there are indeed two traditions of Marcus; the one of Irenaeus,and the one described by Jerome and Sulpicius88. The problem seems to be exarcebated, I believe, by an insistence on a strict literal reading of the chronology in these passages. Jerome is creating in the letter and elsewhere typological spiritual links with previous heretics and this is especially true in the case of Marcus of Memphis, as Virginia Burrus notes, Jerome was intent oncreating a "gnosticized portrait of Priscillian". This liberty is evident in his Commentaries on Isaiah where he quotes Irenaeus regarding Marcus's activityin Gaul, but Jerome extends it into Iberia as well. The use of typological rhetoric and arguments allows for such flexibility and is not necessarily to be dismissed as only willful distortion, although it was not beneath Jerome to exaggerate or invent details. Sulpicius offers a working chronology and links which Jerome greatly exploits in the letter. [p. 382,383]

And I managed to find the original reference of Sulpicius Severus

Chron. II, 45, 9 — 46, 9: 46 (1) Sequuntur tempora aetatis nostrae grauia et periculosa, quibus non usitato malo pollutae ecclesiae et perturbata omnia. namque tum primum infamis illa Gnosticorum haeresis intra Hispanias deprehensa, superstitio exitiabilis arcanis occultata secretis. (2) origo istius mali Oriens atque Aegyptus, sed quibus ibi initiis coaluerit haud facile est disserere; primus eam intra Hispanias Marcus intulit, Aegypto profectus, Memphi ortus. huius auditores fuere Agape quaedam, neu ignobilis mulier, et rhetor Helpidius. (3) ab his
Priscillianus est institutus, familia nobilis, praediues opibus, acer, inquies, facundus, multa lectione eruditus, disserendi ac disputandi promptissimus, (4) felix profecto, si non prauo studio corrupisset optimum ingenium; prorsus multa in eo animi et corporis bona cerneres. uigilare multum, famem ac sitim ferre poterat, habendi minime cupidus, utendi parcissimus. (5) sed idem uanissimus et plus iusto inflatior profanarum rerum scientia; quin et magicas artes ab adolescentia eum exercuisse creditum est. is ubi doctrinam exitiabilem aggressus est, multos nobilium pluresque populares auctoritate persuadendi et arte blandiendi allicuit in societatem. (6) ad hoc mulieres nouarum rerum cupidae, fluxa fide et ad omnia curioso ingenio, cateruatim ad eum confluebant; quippe humilitatis speciem ore et habitu praetendens honorem sui et reuerentiam cunctis iniecerat. (7) iamque paulatim perfidiae istius tabes pleraque Hispaniae peruaserat, quin et nonnulli episcoporum deprauati, inter quos Instantius et Saluianus Priscillianum non solum consensione, sed sub quadam etiam coniuratione susceperant, (8) quoad Hyginus, episcopus Cordubensis, ex uicino agens, comperta ad Ydacium Emeritae sacerdotem referret. (9) is uero sine modo et ultra quam oportuit Instantium sociosque eius lacessens, facem quandam nascenti incendio subdidit, ut exasperauerit malos potius quam compresserit.

So we have the connection for an 'Egyptian' Mark as the head of Priscillianism and then notice the understanding which emerges from various decrees against the Priscillianists. Most notably the one established at the First Council of Toledo that declares:

Si quis in his erroribus Priscilliani sectam sequitur vel profitetur, ut aliud in salutari baptismo contra sedem sancti Petri faciat, anathema sit.

If anyone follows or teaches the errors of Priscillian, or administers a baptism that is contrary to the See of St. Peter let him be anathema.

Take a guess who I think the Priscillian's baptize in the name of ...