Feather duster possible sizes

[fishfanatic2]

What is the chance that my feather dusters will grow large than 2 cm. across? I have like 20 of these ones with white tubes/pink heads. They are probablly hitchikers but they are so small! Thanks in advance!

 

[Len]

Are these guys colonial or are they all over the place? Tough to say cuz there are so many species of feather dusters, some which stay really tiny (few mm across) and some which grow pretty big.

 

[fishfanatic2]

They are spread out over this one rock, and probably many others/I just can't see them.