its probably stressed out. this could be due to bad water conditions. what are its tankmates? is there a driftwood or any hiding place for the fella? just do partial WC and starve it for 2 days. also add tankmates that are very active like SD and tinfoils
do more water change my dat gets dark and hides if i don't do daily water changes but the fish destorye the shrimp insted of eating half the ime...lol i do 3g changes every day on tmy 75g tey doing 15 gallons once or twice a week if you wnt a really happy day