That is exactly what my plants have and I think I have the answer for you. I built one greenhouse and put half of my plants in it. They got these spots on them and I wondered what the hell it was. I have since built a second greenhouse and put the remainder of my plants in it and they all got the same thing the next day.
I think it could be due to high humidity levels? Possible. So I guess I don't know exactly what it is, but I feel it has been ramped up due to high humidity in my greenhouses.
I don't think it is going to be that big of a deal though. The ones in the first greenhouse have had it on the leaves for over a month now and the plants still seem to be doing great.
A fix for the issue (from me at least) is unknown but I can tell you that I do not think it is going to kill your plants.