Crab hitchhiker ID

[NanosAreCool]

I got this little crab on some live rock I got today and luckily he hopped off the rock into the bucket. I was wondering if anyone could ID it and give me some info on it, particularly its agressiveness, reef compatability, and potential to harm small fishes. Below are 2 photos just taken of it inside the bucket. In the second picture the 5 about a millimeter above the surface of the water, particularly the corner above the crab, to give a sense of it's size.

 

[The steven liu]

looks like a coral crab. I had one of those, never really seen it did any harm to my tank.

 

[NanosAreCool]

The liverock is from a friend's tank and he said the only type of crab he ever added to his system besides hermits was a PomPom Crab, could it possibly be one of those full grown and without its anemone pompoms?

 

[Qwiv]

Not a pompom. looks like a young gorilla. I am betting on BAD. Throw it in the sump

 

[DensityMan]

The black claw-tips could indicate that it is a xanthid crab for an ID. Ultimately unless it is a commensual crab (which that does not look to be) it is an omnivore that may prey on any number of organisms in your tank. It may not start out as a problem, but it could become one. Not witnessing a crab rage through a prized colony doesn't mean it isn't slowing it's growth through nibbling.

I'd throw it in the sump or give it away as a mantis treat.