I've done quite a bit of searching r2r and the internet at large. I'm pretty certain they aren't digitate hydroids. The tubes are a bit fragile and don't retract, but will break off if I poke at them. I've ID'd vermatids and spirobids in my tank and these are different....unless spirobids uncurl themselves? Though the spirobids that have popped up in my tank already seem bigger in diameter than these.
I do have a feather duster in my 10 month old tank and an obviously juvenile feather duster has popped up in my Invert QT/frag tank. It was already 2-3 times bigger when I found it though.
I have not seen anything more than a single occasional hair come out of these. Nothing crown like.
If these are feather dusters, do most of them die off as they grow? A natural thinning of the herd? Because I won't have much room for corals if they don't.
Other ideas of what they are? I have checked the critter id sticky.