According to Zosimos (4,25,2-4) M. was a Goth of royal descent (relative of Athanaric [2. 189f.]. He went over to the Romans, received a military command and shortly after (380 AD?) in Thrace achieved a success (probably exaggerated by Zosimus [2. 77f.]) against pillaging Goths. Gregorius  of Nazianzus addressed two letters (epist. 136 and 137) to M. as magister militum. M. was a Christian but not an Arian [1. 388].
1 F. Paschoud, Zosime, vol. 2,2, 1979, 387f.
2 P. Heather, Goths and Romans, 1991.