Looks like a deer tick to me! :lol:
I'd isolate that little bugger, it appears to have one claw bigger than the other, possibly a fiddler crab. They grow too big and do damage so you won't want him in your tank if you have corals. If you have a sump that you can put him in, maybe in one of those fish breeder cages, you can keep him until you find out what he is for sure.