What is this...? Good or Bad?

[Billy1205]

Can anyone ID this thing that growing on my live rock?
Should I remove it or just leave it?

 

[Mr. Bill]

Common featherduster. Harmless filter-feeder.

 

[Billy1205]

will this thing spread around?

 

[Mr. Bill]

They don't get very big, maybe around a half inch or so. As far as multiplying, that will basically be determined by tank conditions and available food.

 

[azcrazy]

filter feeder, they cool to watch and good for the tank