Featherduster worms, also called fan worms, are named for the highly-branched crown, or fan, of tentacles that extends from the protective tube. These tentacles extend from the head of the worm and are used in both oxygen uptake (respiration) and filter feeding. Fine side branches on the tentacles trap small particles of food drifting in the water currents. Large worms may have tentacles crowns spreading 6 inches (15 cm) across. The worms are sensitive to light, water motion, and touch, and can protect the delicate tentacles from potential predators by withdrawing rapidly into the protective tube. The worm produces the tube, secreting a leathery mucus from a collar-like structure at the base of the tentacles. Particles of sand and mud that are collected in the tentacles are incorporated into the tube. (aquapages)
The body tube, which is created by the worm is made up of sand and bits of coral rubble and mucous from the worm. When it is disturbed, it can quickly pull in its radiole into the tube. If it is severely stressed, it will discard its crown, growing it back later. (Saltaquarium)
Small feather dusters can quickly become nuisance pests if they overgrow the aquarium. In my case, the infestation got so bad that I bought a Copperband Butterflyfish to get rid of them.