White hair/crystal structures under rocks

[fixingstill]

About two weeks ago, I found these white hair/crystal(?) structures under some of my rocks. I have had this tank for about six months. They seem to only exist under rocks where there is minimal light. They are stationary, and they don't seem to be doing any harm. Anything that I need to worry about? Please ID this for me. Thanks in advance.

 

[AlSimmons]

It looks like either sponges of some sort or a large cluster of feather dusters IMO. If this is correct both of which are harmless and nothing to worry about.

 

[garygonzales]

yea i got that all over my rocks in my sump..definitely not a sponge wondering this myself

 

[Ron Reefman]

IMHO the tubes are too small and too close together to be feather dusters.