There are two types of crabs this could fall under. Unfortunately, I cant remember their names off the top of my head. And I dont have time to look it up.
WAIT... one of them is a decorator crab. It takes pieces of corals, sand, maybe even small anenomes and "camoflauges" itself with them. Moves around the tank like that. Then there is a "boxing crab" or something like that. It has two small anenomes on it's main claws I believe. Makes the claws look like boxing gloves. But, Im not sure whether the crab "comes with" the anenomes, or it has to find them.
The only real drawback to decorators, I hear, is that they harass your corals by picking at them. That's about it if Im not mistaken.