Petrochirus diogenes!!!!!! Will get 12" and will eat anything it can catch. DON'T KILL IT, they're a TON of people that would love to have the soon to be giant.
River, I think that you are right. I'd have him put it into his fuge. You didn't know that the more expensive the critter was the better it tasted did you . I once had an atlantic hermit that was banished to the lfs after it became a $150 crab ! Wasn't it's fault just doing what it was supposed to do in an enclosed space.
It could be a Dardanus venosus, see if he can get a better shot of his back legs. His claws look more like Petrochirus diogenes but his hind legs make me think Dardanus venosus.