They look like feather dusters to me. But again the picture isn't very clear.
There are 2 white calcareous tubeworms in the the first picture - away from the cluster - those are fanworms = good.
The reason I think the grey clusters are also fanworms is their shape. Cluster dusters have soft pliable tubes - they feel kind of rubbery - they are similar to the giant Hawaiian Feather Dusters sold individually that are much larger. Those small colonial dusters live in colonies.
Do the "feathers" or crowns retract quickly when startled? If the crown retracts but the tube remains where it is, they are feather dusters. If you can pick one or two off and bring them in I can take a look.
Looking more closely at the second picture I'm almost sure they are feather dusters.
Hydroids can come in several different species/shapes, but they often look like tiny button polyps, sometimes they seem "fuzzier". They may also resemble tiny anemones.
This is a picture of a common type of hydroid:
http://www.reefs.org/hhfaq/anemones/hydroids1.jpg/variant/medium" alt="" />
Here is another pic of some among some yellow star polyps:
[IMG]http://www.wetwebmedia.com/Cnidarians/Hydrozoans/Hydropolyps/Winnah_anemones_YAY.jpg alt="" />
This is a photo of a single small feather duster:
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I've looked a bit for a picture that resembles the ones you have but I can't find one... but I can tell you that a tank we maintain has literally thousands of them that look very much like yours - gray or brown pliable tube, and white or brown "feathers".
Jenn