What the heck do I have in my tank?

[Palster]

When I was feeding my fish tonight I spotted what I thought might be a bristle worm then I noticed what I thought was a bunch of bristle worms (I am somewhat new to the hobby so I am not totally sure what a bristle worm looks like) All of sudden the whole thing moved and scurried into a hole in the live rock (scared the heck out of me since I wasn't expecting it to move). I went and got a really bright flashlight and shone it into the hole where the creature was hiding. It was then that I spotted what appeared to be crab pinchers attached to the bristley legs. Does anybody know what this thing is and if it is safe for my tank? I suspect it might be some type of crab. I did a web search and was able to find a type of crab known as a Bristle Crab but there was not much information about it.