I am having the same problem and from what I've read, there are many types of asterinas. One type in particular, Asterina wega, does eat zoanthids. There is really no way to tell them apart from their reef safe counterparts but google Asterina Wega and read the horror stories. I was planning on buying a harlequin shrimp for my tank and then passing on to another reefer but have decided against it. I would be sentencing the poor critter to death if not by starvation, by stress from moving from tank to tank. I just pick the stars off as I see them and make sure they don't break.