It's about an inch long, fully stretched out. I can't find it in my books nor on the web, but I think it is possibly some variety of hydroid, a Plumularia, maybe.
A hydroid is a community. Each circle of tentacles belongs to a separate animal. Looking at this one closely, I can distinguish several structures: segmented "feather" stalks with rows of zooids along the side, each brandishing its little spray of tentacles; a wrapped stem beneath, a few individuals consisting of a stem and a funnel shape, topped with tentacles.
I found tons of interesting photos of Cnidaria (the hydroids' home phylum) on Google; what variety there is in this obscure "rock fuzz"!
- A diver's photo album includes tiny hydroids.
- Pearson College guide to ecosystems. Scroll 'way down to "Underwater at Race Rocks". One of the hydroids.
- Elasmodiver. Cnidaria. The glassy plume hydroid.
- SeaNet, Stanford. Phylum Cnidaria ...
- RaceRocks.com Several hydroid species on a scallop shell.
- A UK site with great photos. Keep clicking on "Next species".