What am I growing now???

I've got a weird plant/worm/thingy growing on my live rock. There's 15 -
30 of them on one piece of live rock, and none anywhere else in the
tank. They're mostly about 1/2 - 3/4" long and maybe 3/16 in diameter, a
bright green on top and white at the base where it attaches to the rock,
and they appear to be segmented.

http://www.markhenryenterprises.com/...ing_021806.gif

Any ideas? Do they stay, or whould they go? Nothing seems to be
eating/messing with them.

Thanks,

mark h