commensal crab or predator?

[Oimate84]

I bought an acropora from the LFS the other week and I noticed what I thought to be a commensal crab. A few days later, after the crab moved from his usual spot, I noticed the area he laid on the acro was white...this a small bleach spot and hasnt spread...Is this normal or should I kick the crab out of his acro since he may be a predatory crab?

 

[drake66]

whats the color of the crab, and physical characteristics, hairy, smooth?

 

[trueblackpercula]

Are the claws pointy or round?
Pointy = bad
round = good