In 15 years of dairy processing and bottling I've never seen that before. You certainly need to have it tested next time you see it. That's just crazy because for mold to develop in fluid milk it needs to be A LOT of mold due to the nature of the bacterial competition in milk. Usually, you will have a psychrophile or general TPC, or coliform if the sanitation is poor. But mold? I've only seen that in cultured products and cheese of course.
Tells me you need to look for another supplier....the air quality in that plant must be horrible, or the way they handle the bottles. I bet they don't blow mold, and they take bottles from a supplier and run them on the line without any treatment. But still, likely to be sh**ty air quality in the filler room.
If you for some reason get it tested and it isn't mold, the other possible source is rubber particles. If the dairy doesn't have the right gasket materials, a good gasket exchange program, including check balls (rubber coated) and PD pumps (some have rubber impeller lobes), the chemicals in the cleaning will wear on these and you can get rubber that smears off into tiny particles.
I worked in a fluid milk plant for awhile. I never saw this. Judging by the pictures I've now seen online, its appears to happen with some frequency. It certainly looks like mold to me as well.
see recall (decrease shelf life?):
https://www.nbcwashi...oncerns/165462/