this might sound wrong but i got that same shit on my plants a long time ago when i veg with cfl to close, thrips will leave a white trail and dots, i dont think thats powdery mildew i have had that. look close to the leaf almost from the side you will se bunch a like almost fur very small hairs look where you se the white and see if those hairs are pushed down or gone thats what i had i moved my cfls up about 4 inches it was gone with in a couple of weeks or it could be the start of thrips i hope this helped talk to some guys at your local hydro shop or compassion club