Starfish

Does it look something like this? Is so, it's an Asterina star.



As Daniel states, some will be nuisance, some not. I've too heard to determine if they're bad, you have to wait it out and observe if you have them in numbers.

Once, after adding a new coral, I woke up at 3am that day, I saw one on the glass and removed it. That's the only one I've ever had. Sometimes I second guess myself, wondering if I should have kept it.

 

well if you ever want some i have what seems like millions of them in my tank. never hurt my corals yet!!