Its not on the new growth and its not on all the leaves. White widow auto 59 days from seed. what could this be 
Looks like it could be the beginning of WPM or maybe some kind of bug issue.
Thrips, maybe.
Check this here for some more diagnostics.
This flow chart may help diagnose leaf issues.
Doesnt look like bug damage to me (it still could be) looks like some kind of early stage deficiency. Or over/underwater. What do your runoff figures look like? What have you been feeding with and what kinda ph and TDS does your nute solution look like? How often are you applying liquid?
I would check ur plants you may have a spider mites or some other bug issue