Could be a sting mark. He may have tried hosting something he shouldnt. I had a clown try to host a maxi mini nem and ended up with several sting marks. Started as tiny black dots that faded to a brown color over time.
It won't be a sting mark because its a FOWLR tank with two clowns and snails. I've never noticed it before until yesterday. I was thinking maybe it's his butt??