I have something that looks like a feather duster but i am not quite sure. Does anyone know what this is. I purchased a live rock and it was already there.
It's kind of hard to tell from the photo, but if you're referring to the white S shaped object, it look more like a polychaete worm. Nothing to worry about. If you see more develop it's a sign of overfeeding the tank. I grew lots of them when I had a heavy feeding hand, the fish always looked hungry to me. I think they learned to beg from my dog:scratchch.
Maybe one more photo , closer and sharper, could confirm it. Someone else will look and I'll be interested in their opinion.
Henry