Is this powdery mildew?

possibly... However, has any water dripped on the leaves? This can be remnants of evaporated water too. Since it is on the underside, i doubt it. it's very small and isolated at moment it seems? should be okay if you start to deal with it now. Fan will help. If leaves are overlapping and causing standing water, remove a leaf or 2. (you want to prefer not to cut leaves off...) Airflow, lower humidity, if not bad for you plants, can both help. RH <55-60% is best to avoid microbial growth... may not be best relative to VPD, but you weight your risks and do what's best for your context... in this case, i'd ignore vpd and ensure no growth since you've seen hints of it. in the past i've just rubbed it off and it went away, but my humidity wasn't an issue either and likely fixed cause of problem when i thinned plant out a bit. clean it off but make sure to use a new wipe each time or you may just contaminate other leaves... Believe milk can work? Check out the GD Copywriting contest submissions.. at least one of them goes over various options for powdery mildew as far as a more proactive approach. Since you are in flower, i'd avoid anything involving pesticides, neem and the like... just because somethign is 'organic' doesn't mean it is safe to spray on flower you will smoke later. neem is not safe to spray on flowers. i don't think it does anything for mildew? more for killing pests by making leaves poisonous.