White spots on leaves, what could be?

Check out my diary, I think I had the same issue, the spots aren't going to go away but to properly wash the leaves use potassium soap dissolved in water and spray it on the leaves, besides cleaning, it kills quite a few bugs that might be on your plant, which is probably what it is and you just don't see it. https://www.amazon.es/Jabones-Beltr%C3%A1n-56058-Quitamanchas-natural/dp/B00X7VU6L2

It looks to me as like you getting some of the nutrient solution on it and its reacting to the light, best thing to do is watch the newer leaves and see if they come out that way, if so then you have a problem that needs diagnostics

i looked for bugs in the whole grow and inspected each one of the plants. I found absolutely nothing

Looks like leafhopper damange to me. check for bugs under leaves. add some sticky traps to see if any get caught.