what is this?

Magnesium deficiency combined with nitrogen toxicity.

Thats a tough one. I dont know which one but pretty sure this would be a macro nutrient problem does not look serious at all I'd just remove affected leaves and carry on incase its some kind of mold. check the bottles of new feeds and see if your missing anything buddy best advice. It could be the starting sign of a toxcity too so i would flush anyway just to be safe. keep eye on it.