Seems like most peoples hitch hiker crabs are less than friendly. If you can catch it now while small and get it out it might be a lot easier than when he grows and get VERY VERY hard to catch. Good luck!
Yeah, with those pointed claws, he doesn't exactly strike me as being reef safe. Put him in the fuge or just get rid of him all together. If you have other inverts in your fuge, he might very well decimate them.