leaf

Looks like mildew. What’s your humidity?

Looks like mold check your humidity level , or add less nutrients water a week or 2 with just water and keep it a little dryer in the tent .

Try to wipe it off with some neem oil mixed with water or just remove the leaves if it dint spread to far, also keep your humidity under 70 % to be safe and check that your airflow pressure is sufficient enough.

Another note on powedey mildew, its a fungus and spreads from plant to plant by spores, the white fur you see is the mature fungus and by the time you See it the spores are already colonizing on the other plants. Fungus likes cool moist environments so lowering humidity raising the temp slows it's growth buying you time to treat your plants. With most products you will need to treat again in 3-7days, spores are swirling around your grow space and can still infect your plants days later. Good Luck!

Powdery mildew by the looks of it from your pic. Lower your humidity below 50% bring the temp up over 75 and you will need treat all your plants immediately or it will spread like wildfire. I use sulfur now, there are other products that work well but I have to follow guidelines and sulfur is acceptable treatment. Some folks will trash the grow and sanitize the grow space it can get that bad. Read up on it.

Powder mildew…get something to spray asap if in veg…mabe a sulfur burner too…get humidity down to under 50%

40 parts homogenized milk and 60 parts water and spray affected areas and repeat as necessary. This will control PM