Does it have a black line in front of its head? If so those are supposed to be ok. I have a white-ish Acropora crab and it has this line, no worries at all.
I think its one of the good kinds of crabs that in a well established sps tank or ocean it would be harmless sort of speak.
I am trying to get the hang of sps so I put him in the sump.
I intentionally purchased a brown sps coral from Max Reefs, the polyps still extend but it was not doing well. I purchased it to gauge myself on sps keeping, I shall try to nurse it back to health and bring its coloration back.
Due to this little experiment I did not want the crab eating polyps and me not knowing if i could not nurse the sps coral back to good health due to say maybe the crab eating more then he should have.