2nd gen white patches

These white-ish, dusty patches have been occuring on all three of the second generation plants since early on, I don't recall seeing this on the 1st generation plants at all. Kind of wipes off a little when you touch it, but you can still see where it was. Any ideas??? Greatly appreciated. Humidity is 30-40% always and I haven't bee spraying them with water because, as far as I've read, you want about 30-40% humidity when the plants are flowering, and 50-60% when in veg.