there are 4 of them in my 55 gallon mbuna tank there is little aggression in the tank is this weird behavior? he eats great looks healthy and co-exists with 4 yellow labs, 4 yellow tail acei 4 rustys any ideas?
If they're recent additions it can take a couple of weeks for them to feel comfortable in the tank.
It could of course be a result of aggression or expulsion from the group. Watch for signs that someone is keeping him there, such as when you feed, if and when he comes out for food, what, if anything, makes him go back there.
He could also simply enjoy that spot.
Acei will swim more top level, but hiding behind the filter is a classic response to aggression. The species you have are fairly peaceful (for Mbuna), but labs and rusties (I've heard) can be spirited. It's possible the hiding fish is female and trying to escape being prowled by males. If he's male, he's getting picked on and you might have to remove him if he doesn't settle in soon.
Watch for it to stop eating, and for torn fins or other injuries.