Its powdery mildew. remove leaves and separate plants. Its environmental factors. Not sure on the environment since no picture of area, but moisture is being trapped in your eco environment. You may have a densely vegetative area, causing insufficient air flow and light to prevent mildew. the evaporating water is causing to much humidity while lacking air flow and light to prevent mildew growth. balance your environment stop watering for a couple days, and supplement more air movement and light. Avoid watering night times or too early prior to light. You are in pre-flower and may have time to recover.
If in hydro, adjust your timer by doubling time between regiments, and insulate top base from plant from watered roots. Your plants look to be soil, but noticed the clay balls at soil.
you can try your luck by grabbing some trimmings, clean, and see if you can re-veg and do later harvest.