This is most likely a Sabellid (Family Sabellidae) polychaete, commonly known as 'feather duster' or 'fan' worms. These are harmless filter-feeding sedentary polychaetes. They have great variation in size shape, and color of their feathered 'crowns'. They typically live in leathery tubes made of sediments and mucus. The larger species with colored 'crowns' are commonly seen in pet shops for sale under the common names above.