Strange algae (?) in my sump

[Dash]

I was doing a bit of maintenance in my sump, taking out some hair algae, and found a strange plant form growing on the rock. This is not one of my original live rock, but one I got from a LFS not too long ago and perhaps unwisely put into my sump. They are upright in the water, and the larger ones look like white segmented worms. I found a couple of photos in Google but no info attached to them.

Friend or foe?