I’ve explained all (or at least the most common) reasons for the attic mold growth in a few of my posts already. You can go through all articles in the “attic
” category (I highly recommend it) or at least this one: mold in attic
The second post I’d suggest is about ventilation and since you’ve mentioned soffit, ridge, and windows (assuming you’re using those windows for ventilation at the same time as soffits & ridge) that might be one of the problems. Soffit and ridge vents should never be mixed up with anything else, like gable / windows, etc. - attic ventilation
Also, seeing the vents installed doesn’t always mean that they are performing well, they might be clogged, maybe soffit vent covers have been installed over the solid board or only partially open / not as much as it would be required, and there’s ventilation balancing issue as well.
As for the plumbing vents, it would matter if any of them are open into the attic, sometimes hidden under the insulation layer.
Cleaning mold is explained here - how to clean attic mold
- since there appears to be a lot of mold you’d most likely need to go with media blasting. However, if you have an older, ready for replacement roof, you may want to consider roof and decking replacement… read the article.
Since there are too many variables that might be responsible for the mold growth in your attic and nothing obvious at this moment (maybe except for the windows as a source of ventilation) there’s no easy answer, you need to go step by step and eliminate each possibility.
Let me know whenever you have some more “data” / after you go through the items in my post. Just click “comment” below.