I have a similar issue, and it seems I saw somewhere that it is a cal/mag def.......

We use different water, I do not have spider mites, it is not a mold to microscopic exam.

So, I only see in during the last 3 weeks of flower. I am trying Cal/Mag now, but I noticed it really dropped the ph of the water and I had to buffer it first. Next time I give less than the advised I think.