if you identify it as Powdery Mildew i would suggest Serenade. It is a bacterial spray that works really good if applied frequently. No danger really in using it, except the potent smell

If you see it coming back, maybe look into a Sulfur burner. I had powdery mildew in my grow room and could not get rid of all of it. It is VERY PERSISTENT. I have not used a burner yet, but it came highly recommended if the room os of a larger size. Good luck!