IMO most species of Asterina are prefectly harmless....and I would say that is the experience of the majority of people keeping tanks who have them (many many many people). I STRONGLY disagree with the statement to remove them. Very few are bad, and it is wasted effort at best...because it is virtually impossible to remove them.
Most will just eat bacterial/algal films, which is why they are commonly on rock and glass. Few will eat coralline algae - but realistically this should regrow....and even fewer, IMO, eat SPS or soft corals. I have never had issues with them eating zoos, personally. Like many animals, they are often unfairly implicated in losses.
Just enjoy unless you see actual positive signs of trouble. But you would be an exception if you did, IMO.
Asterina stars tend to have asymetrical arms and somewhere between 5 and 8. They reproduce easily in tanks. They will not get much larger than a dime or nickel.