GET RID OF IT, QUICK! Blue Crabs are highly predetory and opportunistic creatures. A sleeping fish is as easy as a microwave dinner to these guys. When I started into SW about 15 years ago, I had an aquarium at the beach on the gulf where I lived during summers. I was into taking creatures out of the gulf and putting them into an aquarium - lots of good animals.
Well when I went off to college at Auburn, I brought back a bucket of things I caught for the home aquarium. In particular, some very small blue crabs that I was going to feed the cat shark I had.
Let me just say that the shark evenually won and ate all the crabs, but not before the crabs killed every fish and animal in the tank. Watch them - you'll actually see them try to catch fish.
I say flush 'em or chop them up for a treat for your fish. Either way, someone's getting eaten.