Thats a Swahelian Death Crab. Highly venomous. It has a stinger that will ingect boiling acid into you. Just kidding, Ive always wanted to say that. Anyways this is probably the most common crab ID Ive ever seen. I dont remember the name but its bad. It will eat pretty much whatever it wants.
The fact that you saw him munching polyps says he is bad news no matter kind of crab he is! Sorry, I cannot usually ID crabs, but will say you are ahead of the game by the fact that you caught him acting naughty and got him out.