It’s actually a form of mold
Defoliate the infected leaves
And get more airflow
And reduce humidity to just under 50
Hope this helps as I’ve had the same problem
Happy growing
pH fluctuations will cause similar issues... check the pH of the water/feed going in and check again coming out.
If everything is good, then it also has some similarities of Nitrogen def.
Hope that helps.
Happy Growing!👉😁👈