I just saw this crab for the first time yesterday. It must be a hitchhiker on one of the live rocks that I put in my aquarium. It is about 1" in width. Doesn't seem to be hurting anything, but i'll keep an eye on him.
I don't think that's an emerald crab; looks too fuzzy and it's not green at all!
Does it have any "catcher's mitt" appendages that it extends out to capture plankton and such? I had a couple of crabs that looked like that one that did and they were primarily filter feeders, kind of like a porcelain (aka anemone) crab.
Keep an eye on that one, though; crabs have a tendency to be predatory, towards corals as well as other inverts and fish! I had one that LOVED to eat xenia!! He got fed to my trigger fish...
Thanks for the replies. I have seen this guy a few times now. He seems to keep to himself and doesn't seem to be bothering any of my corals or other hermits or snails in the tank. I will try and get a better picture of him. Stay tuned!