What The...?

omg ... you got one of those ...

is the tip of the claw black? if not ... it looks okay.

Emerald crab? You'll be able to tell by the top of its shell, it would be really ridgid and bumpy.

If the tips of the claw are black, you may have a Chinese mitten crab.

Dude.. I had a couple of these. They came on my rock. After some research I took them out. They say they bore through the rock and may nip your soft corals.

See this link, it helped me. http://www.chucksaddiction.com/hitchcrabs.html

Looks like an Emerald Mithrax crab. I have a couple of these. There cool.

This pic makes it looks like the crab has smooth back legs. The crab that I have, (that I hope is an emerald), has hairy back legs. HTH