What is this?

[Raquiros]

There are some of these in my live rock..

Scientific Name for research purposes? Are they good or bad?

 

[DDon]

Feather dusters (no idea on scientific name). Harmless

 

[Newsmyrna80]

Looks like a hydroid. Refer to this website...good hitch hiker info. http://www.lionfishlair.com/hitchhiker/hitchhiker.shtml

 

[peasofme]

mini duster possibly