They can sting corals and they can spread quickly and take over your tank.theres many different products on the market that kills them.i like the majono wand myself.it zaps them lol.whenever you see one be sure to try and get rid of it
Nasty Critters.... I agree with above peppermint shrimp. Get them ASAP. They get in everything, return lines, overflow, canister filter etc. Pretty much anywhere you have water best of luck to you