Hello and thank you for your inquiry.  It sounds like you may have Asterina starfish in your aquarium.  Have a look at this article by Anthony Calfo on starfish, towards the end of the article it talks about them. 

If they are truly causing a problem in the tank, manual removal is your best bet.  Some people have "trained" harlequin shrimp to eat them (harlequin shrimp feed only on starfish), but if they wipe out the population you will need to start buying starfish to feed the shrimp.

If there is anything else we can do for you please let us know.

Keith M.

Marine Depot Customer Service