This is what it is.
You over fed them.
And the Nitrogen builds up in the fan leaves (starting at the top)
And burns.
It could be from the wrong ph in your water.
It should be 6-7.

It looks like Mag dificency because when you over dose MJ on Nitrogen it locks out the Mag.

Flush it a couple times.
Alot.Flush it real good.
And raise the light a few inches.
Watchfor hermies.
You can hermie them from over feeding.
It takes a good 7 days for the nute burn to totally show up.
So in a few days it will look way worse.
And then start to heal.
Watch those nute levels.
MGrow sucks for MJ.
Get some Black Gold or Foxfarm.