good crab or bad crab?

[bcollie]

I recently purchased a large frag with several mushrooms on the frag. Tonight I noticed movement on the frag, under a mushroom...it was a crab! Not only did I discover one, but two!
The crabs are not hairy or fuzzy, and although it is sort of hard to tell, they appear to be a light faded green with darker green mottling on their bodies and green circles on their legs.
Any ideas what I've got and do I need to be concerned?
thanks, dr

 

[whatsmells]

Kinda sounds like emeralds with that description, if so they are hella cool for a reef tank. they do great work on alge. post a pic if you can.

 

[bcollie]

I got a better look at the mystery crab tonight and the crab is not green, it is blue (sort of a blue gray)! The right claw is larger than the left claw, and both claws are tipped black (or navy blue). Its rear legs are ringed blue and beige(?). Its head is more flat or square than an emerald crab's head.
Now I discovered something else that looks like a clam on this same frag. If all these critters are harmless, and beneficial to the tank, I would say I got my money's worth puchasing the frag.

 

[salty cheese]

Sounds like it may be a species from the Xanthid crab family.
A pic would help.

 

[reese]

I agree-it sounds like an "acropora" crab which I believe is in that family. I just bought one and now cant find him-mine is purple and he has a flat smooth head. If thats what yours is, I know they are peaceful and wont bother your reef