Bubble looking thing on rock

[PurplePolyp]

My girlfriend noticed this thing on my live rock today. It's below the center portion of the polyps straight left of the toadstool head. It's round and matches rock color pretty good. She said its flat but looks kind of bubbly. What is it?

 

[Pryzz]

Looks like a feather duster to me. Harmless filter feeders.