Hi bro, most of the acro crabs are usually safe. They NORMALLY have a mutual beneficial relationship with SPS.
However, i do notice a very few acro crabs that are more notorious and seems to be opportunistic feeder on SPS tissues, they usually are very tiny (not more than five cent coin), greyish dard hairy body and comes with a pair of errie white color eye.
A photo of the crab you suspect would help.