What is that dark stuff on concrete?

1

Hello everyone. New to pressure washing here. I was wondering, does anyone know exactly what that dark stuff is that accumulates on concrete? I mean, like know exactly what it is? Is it mold?

I come from a background of car detailing so I know a bit about cleaning. But that with concrete stains, its a bit new to me.

2

Around my area it’s just whatever is in the air combined with crap in the rain with a few lichen for spice

3

Mold, algae, other living critters. That’s why when you put a good pretreat on there it turns brown and cleans up faster :+1: