What type of siding is that? Is it vinyl? it looks like it could be vinyl clad aluminum siding. If it is then thats oxidation.

I see that a lot on light colored siding. It is worst in rural areas, especially near fields. I think it is dirt or some sort of airborne contaminant. Under the soffit never gets rinsed by rain, and that is where the attic "breathes in" through the soffts, so it collects this fine, really stuck on stuff. I see a barn in the background, so this looks rural. Did dirt run out of the soffits when you got them wet?

You can scrub it off if you get right up to it with a hand held brush or Mr. Clean magic eraser. A brush on a pole unfortunately won't do it. That is one of the worst I've seen it, though. I don't believe it is oxidation, or maybe it's oxidation that has become dirt stained? It is about as hard to rub off as oxidization.

I usually prepare the customer for the fact that the first few "laps" of siding under the soffits on white, yellow, or light blue could remain slightly stained. Usually they probably would not even notice except that I point it out to them. My own house is a 28 year old light yellow farm style house in a rural area with a 2 farm fields very close, and I have this a little bit. I have never seen it that bad though, so I'm sure the customer wants it off. If anyone finds a product that can remove this, I'm all ears.