Hi!!
It is a scar! Trito was an octarian soldier that entered the deep sea metros. He got partially sanitized due to an accident, so while the effects aren’t there, the wound remains (kind of like how people like to give agent 3 the eye scar). He keeps it hidden most of the time, I don’t know if anyone has noticed but I only draw him with his shirt off around Kinoga, who was a fellow soldier and one of the few that Trito feels comfortable sharing it with. Thanks you for asking :3