Many of the Geophagines (and this includes Acarichthys) are social fish, which like to live in shoals with members of their own species, if kept separately they can become either hyper dominant, or extremely paranoid. Your fish is very young, so it is simply showing signs of dominance.
Acarichthys are actually thought to be on the aggressive side, as far as Geophagines go, especially in any tank under 6ft .