I had one of those crabs a while back (year or two) that hitchhiked the same way, and everything I read said beware. Also, If I remember correctly, it's endangered... Therefore I wont tell you what happened to the guy. From what I noticed in my tank, in the few months he was in there I lost numerous snails and he grew noticeably fast. Unless its a crab I can ID thats known reef safe when I dip, it takes a trip to the green belt.