Its a hit or miss.. these are asterina stars, while most are reef safe and harmless as said, some are not reef safe, and 1 can turn into thousands in short time... they are consitured a bad hitchhiker because they can reproduce rapidly like rabbits and in rare cases whipe out corals... Harlequin shrimp are the best solution to rid them but its a matter of whether you want the stars or not... they are super tiny and it will be hard to tell if they are the ones causing damage.