Something growing in Rock

[rahul123]

Since last few days I am observing some white thread like structure is growing in rocks. Maintaining my FOWLR tank over 1.5 years but never seen this before.

Attaching images for reference.
What is it? Good / Bad?

 

[littlefoxx]

Kinda looks like a sponge to me

 

[CallMeChris]

Kinda looks like a sponge to me

Agreed

 

[MoshJosh]

Yeah my guess is sponge as well. My LFS display tank has a bunch of these, and my tank has started growing them (or something similar as well). Harmless as far as I can tell/