If they don't look like a starfish as you stated, I am guessing flatworms. If they go on the glass when the lights are out, then I am really thinking such. Annoying, and you can research remedies to such, and then decide what route you want to go, as people seem to have mixed experiences with most. There is also some types of fish that supposedly sometimes eat them with mixed results on these as well such as 6 lines and mandarins, but I'd try other routes first, and if going the fish route, make sure they are compatible with what you have, as well as that you are able to support the fish (esp mandarins) in terms of feeding and tank size. Also read of accounts where they dissapear just as fast as they appeared and vice versa for no particular reason. Try to figure out what caused them to show up, so you can avoid it in the future. Also try not to break them when removing, they release toxins.