Are the rest of the foliation like that too? Is it just on set spots or is it in the whole plant overall?
While it could be mould starting to form, seems a bit unusual to start in those places, so i would guess it has something to do with nutrition, are you not missing P or K? Are u ph'ing water? Are you using any nutrients, wich ones?(because it may be a nutrient caused nutrient lockout.)
Get back with these answers, lets talk about it.
Hope it helps and if you don't mind chosing answer it helps a lot in grower of the month competition.
Thanks and good luck growmie.