Scientific Name: sp?.
Family:               Annelida
Common Name: Fan Worm, Feather Duster Worm

Description:
Fan worms range from very small inconspicuous specimens to relatively large ones that can extend over an inch across at the crown.  The white ones shown here are some of the larger ones commonly encountered.  They live with their leathery tube buried in the loose substrate.  Other fan worms build small calcareous tubes attached to hard substrates.

Good or Bad?:
Fan worms are never bad.  They exist in virtually every reef tank and earn a living by filtering the water through their fan to collect small foods to eat.  Growth and proliferation of fan worms indicates that there is enough floating particles in the water to support them.  Fan worms can be quite attractive depending on the type.  They also can serve as food for other animals such as Copperbanded Butterflyfish.

Notes: 
Fan worms are not photosynthetic and must catch their food to survive.  Tanks will low protein skimming levels and those which are feed small foods such as phytoplankton will tend to have more fan worm growth.