Hi ScottK ;-)
Difficult to say, could be a non reef-safe crab, so try to collect it and then photograph it in detail, both upper and lower part, so that we may have more data to a possible positive identification. Also by removing it, temporarily at least, will also prevent any possibility of it damaging other invertebrates, should it be in fact harmful for them.
Looking forward to read from you with more detailed photographs
Cheers
Pedro Nuno ;-)