He's cute but with those black tip pointed claws, I would definitely move him out. I had a huge gorilla crab that lived in my sump for a long time until he managed to get stuck in the skimmer intake.
He was really feisty. Pinched my husband, scared me every time I was digging around in the sump, and promptly killed any other crab rejects banished to the sump. Actually kind of miss him. 