Powdery mildew: the white dusting on leaf surfaces that wipes off with a finger and comes back within days. What separates it from the several other white marks on a cannabis leaf, and how it is managed in a room rather than treated on a plant.
Unlike most fungal problems it does not need free water on the leaf, which is why it appears in rooms that are otherwise dry and why growers who fixed their watering are surprised to see it. High humidity at night, poor air movement inside a dense canopy and a large day to night temperature swing are the usual combination.
Two things are worth settling early. The mycelium sits on the leaf surface rather than inside it, so it is visually obvious and easy to test with a wipe, and the same white can be residue from a foliar spray, hard water spotting or the trichomes themselves. And treatment close to harvest is a dead end: anything applied to a flower that will be dried, inhaled and possibly tested is a compliance question as much as an agronomic one.
Where to start
Related elsewhere on the forum: Integrated Pest Management for Cannabis in Diseases, Insects and Mites.