That there looks like powdery mildew and you need to either wipe it off the leaf or remove the leaf. Assuming you are in veg I would spray with neem every 3-5 days until flower to hopefully prevent any more spreading
Yup powder mildew, dealt with it before but can be a bitch if left unchecked. Neem oil like said above and good luck Sent from my LGMS345 using Grasscity Forum mobile app
Clean the hell out of your grow area after this cycle, keep humidity down and have really good airflow. My current cycle had it and I poorly decided to think I had it under control before flower just by removing leafs and it exploded in flower... It's much harder to work with in flower so get that neem going man.