I stared at the slowly curling tentacle for awhile, which eventually pulled back into the hole, and went to the internet. I had tentatively ID'd it as a micro brittle star living in my rock (good) when I happened to look over at another section of rock, noted some small white tentacles, and watched in mild amazement as a tiny white brittle star pulled itself out of its hole and strolled a few centimeters along the rock.
So I have at least one, possibly more Brittle Bobs in the tank.
Man, live rock is so awesome. Three weeks, and I'm still finding new stuff in there.