Hydrogen Peroxide
@3% mixed with water ( 1 part H2o2 to 3 parts water ) can be used to control powdery mildew, but usually wont get rid of it completely.
I recommend using Potassium bicarbonate, it is a contact fungicide that kills mildew spores quickly, and it’s also approved for organic growing.
1 tbsp per gallon of water. A single spray should get rid of it, But do a second one 4 days later just to make sure.
Prevention is key, If you get an osculating fan and place it on the floor of the tent under the canopy slightly angled up( only 10 degrees )
And a second stationary fan going above(about 5") the canopy. Then you should have enough air circulation to stop the WPM from forming.
If you have good air circulation I would check the underside of your leaves and branches for bugs( WPM hits sick/weakened plants )
Happy Growing