Hello, I have several of these whitish, transparent "worms" that extend and retract and hang out in the current. They do not seem to have a hard spiny base, they just extend directly out of the live rock. This one seems particularly irritating to the zoas. My son thinks they are hydroids. Hopefully not...
I think they're "something else"... and a harmless something else at that. I just checked my reference books (where I recalled reading about these) but did not find anything there. I'll keep looking and will post whatever I find.
I've had these for years. Never more than a few here and there.