I have a harlequin in my 65 and I bought it to eat those stars about a year ago or so. My glass was covered with those little asterina stars. I had heard that they killed corals, but I just didnt like the way they looked and they WERE breeding extremely fast.
Within the first day of the harlequin being in the tank he was picking up the stars and eating them like little star shaped cookies. It is really cool to watch. But then, like Woody said once the small stars are all gone he will want chocolate chip stars. But I knew that going into it. Mine will eat one large one per month or two small ones. If I get two small ones I just put one in my refugium and then fatten him up with silversides before feeding him to the shrimp.
He can smell the stars as soon as they are put in the tank. He immediately pops out and starts dancing until he can get his hands on it.