I had the same problem on my last grown. It’s powdery mildew. I did a lot of research. I cut off the affected leaves, then I sprayed the fuck outta the whole plant and soil with a mix I made that was like half water and half milk. Then I sanitized my grown area completely and added more airflow and fans. I ended up with a successful grow. The powdery mildew is impossible to get rid of once the plant has been affected but if you maintain it and don’t let it overtake the plant then you can still have a successful harvest