I haven't been able to find a good match but it is in the family Xanthidae. Even if we could find the genus and species we still might not be able to find out what it eats and whether it's reef safe or not - which I assume is your real concern. Most people yank xanthids just on principle. If you like it just keep an eye on it - especially at night - to see if it sticks to detritus & algae or goes after your corals.
This has been my opinion on hitching crabs. If the pinchers are blunt and round it should be safe which means it is a algae eater. If the pinchers are pointy and sharp, chances are its a meat eater. And to get it out.