After that hitchhiker thing sat there for a couple of days without moving, i decided to touch it with something to see if it would move. It was just a shell, so I figured whatever it was had died in the tank. Not so! Tonight while sitting watching my tank, I saw something come from under a rock cave and nearly scrared me to death. All of a sudden I see these hudge claws, then the rest of this thing comes from under the rock striaigt for the seaweed salad that I had just put in the tank. I jumped and he scooted back under the rock. I sat there for about an hour waiting for him to come back out so I could take a picture, but nothing. It looked like a large bleached out emerald crab. It definately had some big a** claws.