That looks like thrip damage. Picture of a thrip attached. They're tiny little bastards but they leave small brown drops called "honey drops". The arrive with no fanfare but they can ruin a crop very quickly so act now and ensure you kill all of them.
I have no idea where they came from ("I musta got it from a toilet seat, Ma!"). My grow is in a garage with a door out to the back yard, which is a strip of gravel 15' wide that runs behind the house (typical SoCal "garage with a house attached"). A couple of plants growing around the AC unit, nothing more.
After getting thrips last summer, this year I was
very careful to rarely open the back door and to only have it fully open when I walked through it. I opened it to pump out the res, to carry a trash bag every every now and again, and to wheel the trash bin through the garage every other week. All in all, it's open just a few minutes a week. Despite that, I got hit this year, as well. No more summer grows for this kid + I'll be using Spinosad prophylactically for my grow next spring.
From what I've read Neem is the go to for thrips. I got hit in late flower last year so I couldn't use Neem but went with Spinosad instead.