I don't think all asterinas are damaging. I have plenty of olive green ones that plainly eat only algae. The blue and lavender ones I have repeatedly observed over a period of years engulfing zoanthids at night and leaving them shrunken, yellowish beige, slimey and soon dead. I remove that type on sight.
There are probably numerous species of white asterina, some of which might be likely to eat coral. I had a linkia that ate coral, it was blotched baby blue with rust spots. Unfortunately it is almost impossible to identify the exact species of most starfish, and many have not yet been formally indentified. Of those given a genus/species name it's likely few have had their diet formally studied. So basically it's hard to say, and we have to observe what we have ourselves.
Since a lot of predators are nocturnal it's worth taking a flashlight to the tank at night now and then, and especially checking any colony having problems, never know what you will find. That is how I busted the stars that ate my zoas years ago. Also got a reef spider.
I have a friend who tosses any invert he can't ID, and I think that is a bit extreme. I have never seen the green asterinas do anything but make bite marks in the algae on the glass, if yours do the same you are possibly ok.
I am so reassuring eh?
I hang out with people who being scientists have peer pressured me into not making assumptions without proof, which I can't do without the letters after my name and a whole lotta training and lab gear. So I tend to err on the side of caution.
What I would like to know is if anyone has ever submitted a zoa pox infected specimen to a coral pathologist? If not maybe it's worth the effort. Fish pathologist near me is quite reasonable, maybe I can find a coral guy to help us.
Kate
