Is this powdery mildew?

It does look like it to me. Big temp fluctuations can cause it and some plants are just really susceptible to it and others can be next to a plant covered and not get a spot. With WPM prevention is better than cure. Use Trichoderma when you plant and transplant to help the plant build a defense system. Once you have it, it is pretty much impossible to get rid of. You can use bio sulfur or copper soap in veg to try knock it back, but it is inside the cells of the plant and will come back. Do not spray anything in flower, it may knock the WPM back, but you will end up with rot and WPM, so hit it as hard as you can until you're around 2 weeks into flower and then all you can do is hope it was enough and try keep environment stable, but outdoors, you just need to pray. 🙏