This is caused by too much humidity and not enough air flow. Try moving plants further apart and not growing strains that are too leafy which have more chance of growing mold. Carefully, wipe mold off leaves with a wet paper towel (disposable to prevent spread of spores and wet so spores stick instead of fly), careful not to shake mold spores onto other leaves, with water and spray them down with SM-90 or if not available, neem oil in a 1/5 dilution. Other products would be milk and water in a 1/9 ratio, baking soda with 2 tablespoons per gallon of water, 35% h202 hydrogen peroxide 1 teaspoon in a gallon of water. It is safer to throw out buds that have been infected opposed to smoking them because it is harder to remove all the spores as opposed to on the leaves.
This is usually reversible unless you just wait entirely too long and it takes over the plant, so be sure to watch out for furry white spots. Young cannabis plants thrive on humidity, though this is perfect grounds for mold as well. It may be necessary to buy a dehumidifier if you live somewhere such as Florida. This is especially important during flowering because humidity above 45% supports rampant mold growth. It is extremely hard for mold to grow in rooms with higher air flow because the tiny spores just blow away. It is likely that mold won’t be a problem if you have food air circulation even in high humidity. Also, if the air is not being replaced, you are making it much easier for mold to grow since spores are essentially just collecting. Leaves that are in contact with each other are likely to form moisture, so train the branches apart from each other or defoliate some leaves, especially those completely shaded from grow light which are just using extra energy of your plants at this point, decreasing your yield.