What does it feel like when you touch it? Is it collecting in scratches in brute or is that pattern just random? Looks like it is a scratch gathering whatever particles are swirling around the container. I know Reef Crystals has something added to coagulate impurities which then precipitate out, looks like a dusting of brown on my bin but we may have different flow patterns/pumps, etc. I clean it out every few batches of water and let it settle before pulling water out. Didn't think IO has that added because its not sold for reeftanks, but maybe it does now or it is just in the current batches. Only the fancy salts are Pharmaceutical Grade clean.
If it was mold, you would have stains in the brute above your current water line from when the water was at a higher level, so I suspect its a particulate in the water. Check your impeller just in case it is grinding itself up, but probably salt impurities. If you don't clean the containers, you end up with a crust everywhere eventually.