All Asterina are not created equal. There are many different types with different eating traits. Telling one type of Asterina from the rest is difficult at best. Typically I take a wait and watch approach. But if you start to see a population increase intervene in a timely manner. One method of Asterina reproduction is to simply split, leaving legs behind.
Get it out now while you can. Once they begin splitting you will never be rid of them. I have hundreds in my system and while most eat only algae ive lost some zoas. They are like weeds in a garden, not all bad but not all good either.