I can honestly say that I've never ever seen my Asterina stars bother SPS. I remember years ago being warned about them, get them out ASAP, etc., but I figured, I'm going to wait and see if they ever do any damage. Despite numerous other things which of course DID do damage to my SPS, I still think that they never hurt any of mine.
But the warnings come up from time to time, so there must be something to that too.
My take on it is, there are probably several species within the Asterina genus that look very similar, to my eye I wouldn't know the difference between them. But some are harmless, and some are not. I must have some of the harmless ones.
So I guess all I want to say is, don't assume they're harmful (or I guess, don't assume they're harmless). "Observe and verify..."
Just my $0.02
