These feathery structures are modified
prostomial palps (mouth appendages) of Spirobranchus giganteus polychaete worms, often called Christmas tree worms. These spiral radioles are lined with cilia and are used by the worm for feeding and breathing. These worms have a mutual symbiotic relationship with their host corals. The corals provide structural support and protection, while the worms enhance water circulation which helps corals feed.