Depends on the crab, but yeah they can wreak havoc on corals and fish. Even crabs we deem as reef safe are not always reef safe some more often than not go rouge. Some crabs are adept at catching unsuspecting fish while others just wait for a fish to sleep. I have seen emerald crabs destroy lps corals because they want the food the lps was given others just decide the lps is tasty. Typically if the claws are pointed they are not "reef safe" if the tips look like little cups or are flattened then their primary food is probably algae, but all crabs are opportunistic so caution is warranted.Thanks! Did not know about that- coral hithchikers on live rocks near the surface.
As of the others, mantis yeah, I will get rid and set free on the sea. But are crabs really that bad? Should I get rid of them too?