what kind of "crab?"
crabs are scavengers and will eat anything they can find (including slow or ill fish)
and just about any scraps they find lying around on the bottom
but do not rely entirely on the little guy living off of scraps
try to feed him directly once a day (or every second day)
they will enjoy sinking shrimp pellets (or any sinking pellets with a good source of either veggi's or protien), frozen bloodworms, and even the occational fresh veggi's such as lettuce or cucumber (pinned under a rock, or on a low veggi clip)