It's called filiform corrosion, do a search for it here on the forum. When I stripped my panels I found it also, mostly above where battery box was. Try another panel, maybe the damage was limited to one area.
It will polish out with some effort.
My panels had also been prepped with something that left swirls of discoloration. You can see a pattern of the washing swaths. I'm leaning toward re-zolotoning now.
Shelly : TAC NH-6 | AIR 41359
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