What it's this ?

Looks like powdery mildew, which forms when plant foliage is dry, lighting is low, temperatures are moderate and there is high humidity and poor air circulation or a sick plant. If its only one leaf I would carefully use a plastic bag and remove it from the plant. Since its a fungus you don't want to be disturbing it and spreading the spores. Then treat it. I have read many different methods of treating and everyone prefers a different method. When the plant is budding then its even worse as you don't want to be spraying your buds. Some methods I have read are spraying the plant with( usually requires multiple applications). - H202 at a 3% concentration. - Milk ( 1 part milk, 2-3 parts water) - Organic Fungicide sprays - Neem oil <- Never use on buds!!!!

Good afternoon Doudou3D, It is indeed mold. Before reducing the humidity in your grow and improving the airflow, I would personally remove the affected leaves very carefully to avoid contaminating the others (depending on how big your plant is, of course) and apply a fungicide (during the times of lowest humidity in the day). I would also keep an eye out for the same symptoms in other plants sharing the grow space and apply the same method mentioned earlier. I hope it's nothing, buddy. Good luck and stay strong 💪

Hey that’s mold. I would check your humidity levels as if they are pretty high it will only continue to spread.

WPM... white powdery mold. I use hydrogen peroxide water mixture and spray the leaves (google for different uses) Going to need better air flow. I use a 8" oscillating fan.

Mold....