Powdery mildew. It will eventually take the plant down and there's nothing to do about it...except plant a better variety of squash next time. Multipik is bad about getting it. There's about a thousand different yella squash varieties out there, pick another with some PM resistance and plant again. I plant squash all throughout the summer, it'll make a crop in 30 days flat in the dead of summer if you can keep it watered.