If I get a phone call at 1am from a passionate (ahem) customer about "white stuff all over my plants", I have a scripted stand-by answer I always give. And it always works.
"Your plants have the equivelent of athlete's foot. They aren't going to die of it- but there might be some uncomfortable moments. You know how conditions have to be just right to get athlete's foot? It's the same with plants. Just like with athlete's foot, sanitation is crucial right now. And of course there's a little something we can spray on it to provide some comfort."
It works, every time.
P.S. Hort hubby has come up with a term for when you don't know what's wrong with a plant, but it sure sounds like you do....
"I'm sorry, it seems your plant has spidery mildew"
You know you want to borrow that.
Can I get a little love in the forum?