all it takes is one to turn into many but I've never had any harm come from them most are film algea eaters supposedly the colorful ones eat coral but I myself have never seen it
Those can come in any coral, rock etc. The problem is that they'll much on you LPSs. I've cought a couple of them trying this. Nothing to be worried about. Just keep and eye on them and everytime you see them remove them. That's my advice.