im not 100% sure but he could be trying to itch on the shell if ur tank is sick or he or she is starting to get ick if not then thats a hobby to ur hippo
they have a playful nature from what i have seen, mine picks up smaller hermits and carries them around the tank and then drops them
maybe yours is just playing around :happyfish
Does your tank have alot of liverock or at least place for the fish to hide. My regal blue tangs lay in the rock work all night and hide there during the day.
