Can anyone indentify this white fuzz?

[Tony757]

So i found this white fuzzy looking thinga on my rock where it is in dark areas... should this be a concern ive tried researching it but idk if it is a sponge or fungus

 

[Brad Miller]

Is it spongy feeling when push on with a rod ?

 

[lapin]

Looks like some of the white sponges I have in my rock. Dark areas with high flow