What is this growing?

[superman1950s]

I was wondering what is growing on my LR.

Any thoughts would be greatly appreciated.

My setup is a 38gal hex with HOB power filter and a 200w heater.
I also have about 8-9lbs of LR

 

[ccCapt]

Looks like a sponge.

 

[melosu58]

Moved to SW ID forum