Apparently there are multiple species of Asterina, some are coral munchers and some arent. And it's damn near impossible to verify which species you have. As Calappidae said, they can quickly divide and cause an infestation. Some of my local club members have horror stories of them bent over their 125g systems with tweezers plucking them one by one for WEEKS. Or you can get a Harlequin shrimp to devistate the population in about a week. But then you need to suppliment the very starfish you tried so hard to eradicate to keep the shrimp alive lol. Either way, in most systems they are completely fine. I've had them in every tank I've owned for years and never seen an outbreak.