Finally found him!! Hitchhiker Crab ID please
 

I've only ever seen his big black claws (with white tips) poke out to grab a pellet or some hair algae twice in the year that I've had the tank going. I'm not sure if this is his body or just a moult so I figured I should get some input. The closest ID I've found is a Xanthid Crab but it seems quite similar to an Emerald. Any ideas?
http://i1182.photobucket.com/albums/...k/9764ec27.jpg

http://i1182.photobucket.com/albums/...k/fccc8f81.jpg

Looks to be similar to an emerald crab (likely same family). The flat tipped claws suggests that he's an algae eater and not a "Imma gonna eat yo fish" crab.

keep in mind that emeralds aren't safe either.

I think it's a Mithrax crab. I have both a Mithrax & Xanthid in my system. I believe a Xanthid has somewhat pointier pincers than a Mithrax. Over the years I've observed two larger & a couple of smaller Mithrax in my tank. From what I can tell they haven't harmed any of my healthy fish or other livestock. When my BTA was fairly new & sickly, it would try to take scraps of food I was trying to feed, but that was years ago. Healthy nem now & the crab stays away. One of the larger Mithrax appears to have been killed off, probably battle between the two larger ones, since each was missing some body parts a while back.

The Xanthid on the other hand, I think is responsible for chewing up a Ricordia frag I had just placed on the rockwork last week. Less inclined to think these guys are a good thing to have in a reef than a Mithrax. In the end, either could probably do some damage, but for the most part, Mithrax is beneficial as an algae, coralline grazer & food scrap cleaner upper.

? and what are you basing this info off of?

crabs is crabs.......when they are small they usually stick to algae.....when they get bigger they become opportunistic......some have been reported/blamed for eating polyps and even taking out small fish.......I personally have never seen this but I always live by this motto......"if you didnt add it, you likely dont want it"....