What is this white stuff on my live rock

[TerryLowe]

I have several patches of white that seems to be growing in my tank. Some look like "slime" and other areas look like white cotton balls. What is this and is it harmful?
The patch on this first image covers several inches. The others are smaller

 

[Tahoe61]

Sponge and harmless.

 

[Reefing Madness]

Sponge and harmless.

+1 .

 

[TerryLowe]

Sponge and harmless.

Thanks!!

 

[TerryLowe]

+1 .

Thanks!

 

[randall zenker]

i have it too where dose it come from i haven't added any new rock in over a year