Although they come in many sizes, featherdusters are a popular addition. Larger ones called Hawaiian Featherdusters are very common. A worm resides within a tube, and extends a crown of "feathers." These are used for filter feeding. Although they don't need light to live, feather dusters quickly retract within the tube, pulling the crown in with them when they sense danger due to a sudden change in lighting or current. Feeding phytoplankton every other day suffices.
If you see the crown blowing around in the tank, the worm may have throw it off in an act of self-preservation from a predator (rouge crab?). It is possible for the worm to regenerate a new crown of feathers, but this takes time and the worm may not live long enough or get enough food to regrow. Protecting it from further harm would be a good plan, but there are no guarantees the animal will make it.
Featherdusters are Reef-safe.