I have heard many stories like these. I used to think that the people were wrong and that the asterinas were just cleaning up the already dead stuff. I really didn't mind the white patches on the coralline. But, like I said I had a really nice frag of zoas - about 5 polyps, and I watched them disappear one by one. I moved them around and one morning, I watched one of the asterinas move towards the frag and climb on a polyp. I then got a Harlequin shrimp. I loved that guy and I think they are a great addition to a tank, but you really need to ahve a big enough tank to support one - they are obligate feeders and they only eat the tiny feet on starfish. One LFS recommended I get (very cheaply from them) a chocolate chip starfish and freeze it. Then feed the shrimp a leg at time.