Squash gets powdery mildew. There are any number of anti-fungal treatments you can use, starting in the kitchen. You can mix bicarbonate of soda in water and spray it, or buy potasium bicarbonate. That was the first product I used on powdery mildew when I started organic about 10 years ago.
Today, I'd start with straight 3% hydrogen peroxide (grocery store strength) and put it in a spray bottle and spritz them when you would do your spraying or foliar feeding. It may not make the mildew leave the parts of the plant it is on, but it can help the spread. Other things help like cornmeal (making a tea with cornmeal then spraying it on) or neem. Careful with the neem in hot weather, it can fry plants if you do it when it's warm.
For more, see the library page about powdery mildew