Black Crab?

I'm not sure if I should post this here or somewhere else, so if it is in the wrong place, just delete it!

I have a crab in my tank that I have yet to identify. He has the same body shape as an Emerald crab, but he is jet black. He is smooth and shiny. He is nocturnal and I very rarely see him (whoever gets that rock will have a treat!). I was given him by friend who simply called him a "Black Crab" but I wanted to learn more about him. I believe he is about 2 inches across, but that was a couple of years ago, so he may have grown if he was not full sized.

Any ideas?

Thanks!

Impossible to tell without a picture. There's lots of crabs out there. Would need to see claws and head to have a guess but probably just a cleanup critter if nothings ever gotten eaten or disappeared in the tank. Some crabs get to be a problem once they grow up, eating corals and smaller inverts. Do you ever feed it? Only crab I've dealt with was a little hairy one that liked to take chunks out of my leathers, had to dismantle half the tank to chase him out. That wasn't fun. Maybe yours is a variation on an emerald and a algea eater.

I have fed him Shrimp Pellets by dropping them in front of him when I noticed him peeking out a bit, but he very rarely comes out. I assume he is not a carnivore, or at least not an active hunter as he has never eaten any of my fish or inverts.

I will try to get a pic when I get down there for Thanksgiving, but honestly, I don't think I will even get a chance to see him. I have only seen him maybe a handful of times since I got him from the friend.

I would love to have him and the red hermit if you could catch em Let me know