I scraped the front of my tank and then ripped out a huge devil's hand leather (pretty well tore it clean off its base)... I don't think my clam appreciated it!
In my 8 years of reefkeeping, I have always had asterina stars. I have never once seen them in contact with any coral tissue (sps, lps, otherwise) and have never seen evidence (anywhere) of them eating coral. They actually tend to hover where algal films are thickest (like the clam shell) and seem to get higher in number when algae goes up.
The whole "eating coral", IMO, is just an easy excuse for people when their corals deteriorate for other reasons.
(As an aside, I gave away over 100 of these 3 months ago, and the numbers have rebounded... I would daresay that in my 200g tank, I have over 250 of them (including sump and overflow).)
I second the notion that they're algal grazers. I used to have hundreds of them in my 125, but now that my refugium is growing algae like mad and sucking up all the nutrients, film algae is way down in the main tank, as are the numbers of these little guys.