Sources and Historical Note
The sole substantive record of Mark and Mocius is the brief stub entry in the OCA synaxarion for July 3, which lists them among the day's commemorations. They are not treated in OrthodoxWiki, the Prologue of Ohrid, or other accessible Orthodox reference sources, underscoring how little is preserved about them.
The martyr Mocius commemorated here should not be confused with Mocius (Mucius) of Amphipolis, a priest of Macedonia martyred under Diocletian and commemorated on May 11; that is a distinct saint with a separate feast and account, unrelated to the July 3 pair.