It's a type of mithrax Crab from the look. If you can get a good look at his pincers try to see if they are rounded tips if so it's related to the emerald crab. But like all crab species they are scavengers and will get what ever they can. Is he black/white colored? If so i got one a long time ago that never harmed anything. I tried to get him out. He never fell for any of the traps i put in lol. I would just monitor to see if hes causing any issues. GL
Definitely some type of mithrax crab. I got a red mithrax but ive heard of black ones. Not sure if thats just the picture but I wouldnt be that concerned.