A type of mithrax crab.
Id get rid of it.
I only trust porcelein and acro crabs. For now...
I had a black/brown mitrax crab very similar to yours that broke the tips off my sps to use to build himself a cave... Never knew where the tips were going until I setup a nightvision webcam and found him cutting off small tips and carrying them into a hole...
Gave him to someone who wanted him for food.