The crab looks to be a juvenile so it is a little hard to tell. My best guesses would be either a White-fingered Coral Clinger (Cymo andreossyi), a Warty Reef Scraper (Pilodius areolatus), or a Spined Hairy Crab (Pilumnus terraereginae). Either way, I agree MagicJ. Probably best to remove it to an area where it can live hapily and not cause any issues with your corals.