Thank you for the illuminating footnote: "While “Mýs” is typically translated as “Mouse” (assumed to be a diminutive) it could equally refer to his region of origin, perhaps Mysia. Most slaves in ancient Greece were foreigners who had been captured, sold, or imprisoned."
It seems Mysia was in the area of Lampsacus and Cyzicus (we know Epikouros sent letters to friends in Cyzicus, at least, P.Herc. 1418, col. 7).
According to Strabo (Geography, 13.1.19), Epikouros associated with "the most distinguished of those in this city [Lampsacus]" so maybe he did pick up Mýs around that time and location.