Looking for an ID. I have a mystery crab in my tank and have had him for quite some time. I believe he hitchhiked in on some live rock, which means I've likely had him for over a year. Last molt I found was too small to grab or photograph, but this time, his molt is almost a half in across. I've only caught glimpses of him, and I have not noticed or observed any damage as a result of this crab, but I'm still curious as to what I'm dealing with here because they only get bigger. Any thoughts on what this particular crab is?