My guess is stone crabs. Are they a brownish color? I can't really tell from the pic...got really lousy eyes
. It kinda looks like a mithrax in the pic, but I really doubt that's what they are...you usually have to buy those. If they are brown, and are stone crabs, then they are not reef safe. I doubt they will cause a lot of damage while small, but I'd try really hard to get the larger ones out.