I didn't pay for him or know where he came from. The only this I can think of is that he was a hitchhiker on some dry rock.
Colt, It looks like a Gorilla Crab to me. Black claws=bad. Banish him to the sump until you know for sure. John
Can you get a better picture with his claws extended? I agree with John, the second picture you can just see a glimpse and it looks like a predatory claw. If the claws are pointed it's a predatory crab, if they're shaped like shears they are more likely to be reef safe.
After using my extensive google research I do believe I have to agree with you two. Here is one more picture I couldn't get him to extend his claws but I took a picture of his underside. Let me know if you are having second thoughts on the original assumption. The claws aren't exactly black, but brown, at least you can see the shape of them though.
He could've come from live rock or large frags that you've purchased. I've had things that I've found 6 months after adding live rock to my tank. They can go unnoticed for large lengths of time.