Tidepool Geek wrote:
Spirorbis is a sedentary polychaete that builds a tightly coiled
calcium carbonate tube.

Good guess, but no. They really look like very small barnacles. My
mandarin had a great time eating them, and putting up a dust cloud in
it's wake. I don't see any right now, but all my rocks are still coated
with tiny white circles where they once were.

--Kurt