Small 'feather dusters' growing in live rock?
 

Can someone tell me what these are? I found them in some live rock, they are about 1" max a full spread, and while the lights were not that great, they appeared to be lightly coloured.

Charles

The Feather Duster, also known as Giant Feather Duster, has a fan-shaped crown (radiole) that is tan or orange with brown band colorations. The radiole of some species from certain locales may be up to 7 inches in diameter, or of brighter colors.
The Feather Duster uses the radiole to filter out small floating organisms, which it directs toward its mouth, located at the center of the crown. It protects its segmented body by secreting a parchment-like tube to cover itself. When it is disturbed, it can quickly pull in its radiole into the tube. If it is severely stressed, it will discard its crown, growing it back later.

The Feather Duster is usually found in the coral rubble at the base of the reef aquarium, with some of the rubble being incorporated into its parchment-like tube. It needs to be housed in a well-established reef system as it is dependent upon plankton for its diet. It is very intolerant of copper-based medications.

Supplemental feeding of phytoplankton and liquid organic foods should be provided.

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That being said I currently have 3 feather dusters on my reef tank. Two are Hawaiian feather dusters and one came on a piece of live rock. The others I added myself as my wife enjoys the different colours present on the fan. They are pretty neat as I thought 2 of them died, however the worm just left the tube and setup shop on the sand in a new location. Sometimes they will move if they are being bothered or lose their crown if being nipped at. You will often notice the crown missing and a couple days to a week a smaller crown has been formed. A lot of people will find these as "hitchhikers" on live rock. In my experience they have not caused for any concern.

Although they may be related pretty sure op is talking of the little mini ones that can become almost a pest because they can take over your live rock. I've never had a issue but i have seen tanks with hundreds of them angels and butterflies will eat them as there a nice little snack.

I had a bunch of these hitchhike in on some rock. I liked the way the looked but they did spread uncontrollably. Then I got a copperband butterfly, and they disappeared very quickly, never to return.

There is no tube, like with true feather dusters, these ones are actually inside the rock itself. They did not seem to be bothering the corals.

Charles

Any chance they are Hydroids rather then feather dusters?

Nope, definitely not.

Charles

Interesting... well I'm glad you got a handle on it.