Mildew or trichomes?

I don't know of WPM growing on the underside of the leaves, making me think it is something else. If it rubs off there is a high chance it is WPM though, so try rubbing it off an see if it stains a black sock/cloth with white powder. I would say it is not WPM and its just the fine hairs that grow out of the cannabis leaf. The technical term for them is trichomes but we also know trichomes as the little sticky bulbs that contain all the goodies of cannabis plants so I will call them hairs for now to avoid confusion. I have had plants that are quite hairy its just something in its genetics, helps deter pests and avoid frost according to one site. Squash plants have lots of these and bugs love them so how well it works is beyond me. Keep an eye on it and track if it is growing. WPM tends to operate in spots of locations not really evenly over the leaf like these photos are showing. As for feeding once a week with water in between is a good plan with liquid nutrients. Ensure you are monitoring the EC and PH of the feeding solution to avoid other potential problems. Best Of luck!