what is this white worm looking thing?

I have a couple of these from a LR i got from a LFS. They are white and they extend out of the rock, you can see it on the bottom of the rock near the sand. are they harmless?

 

i believe feather dusters, good things

 

thanks. my initial thoughs were tube worms. they weren't there last night but showed up this morning.

 

do they look "hard" with an opening at the end? if so , hopefully the duster part will emerge soon

 

yea, they do. they look like a hard white tube sticking out of the rock. Thanks a lot.