Grey/White "Cotton" Mat in Bottom of Weir

[mvbrandt]

Weird stuff here. I'm almost certainly just going to vac it out at the next cleaning, but I thought I'd just do a double check. This is at the bottom of the Weir overflow on the tank.

Opinions?

Tank params and behavior otherwise are near spotless. Relatively low nutrient system.

Total space in the photo is a few inches square, for scale. Little nubs are extra small hatchery pellets (get sucked clean every few weeks).

 

[lapin]

Old pizza dough or sponge