I have had one tank years ago that ran with thousands of asterinas, and they never touched a thing....until one day I found them on the palys, and as I removed them off of the closed polyps, underneath them were little bite marks. I bought a harlequin shrimp and within 2 months the population was virtually nil.
I never saw this before that, and never since. Generally they are completely peaceful algae grazers, but on rare occurrences, they can turn carnivore.
What sucks about the cure is that you will need to feed the harlequin starfish to keep it alive after the asterina population is gone. Either that, or pass it on to someone else.