I had one similar too. It was the first one on this link: http://www.chucksaddiction.com/hitchcrabs.html
I gave him away too. Felt bad killing him since it lived in cycled rock with no light or food for 6 months. I had no idea it was even there. I guess he ate sponges, etc... on the rock.
A good way to tell in the future is if the claws come to a point no good. If they are rounded then probably ok, but even then when they get bigger, they can become a problem.