Do you not have a diary for this grow? Where are these leaves from? The top of the plant? The bottom? Middle? This looks for all the world to be a manganese (NOT magnesium) deficiency but I would refer you to this article that might help you decide positively whether or not this is the problem. https://www.marijuana-seeds.nl/blog/manganese-deficiency-in-marijuana.
If it is, it's difficult to know exactly how or why this happened... are you letting your soil dry between waterings? This is apparently one of the consequences of not doing so (along with a lot of other problems overwatering causes)... I would also advise you to calibrate your pH pen.. I'm thinking it's out of whack - double check it just to be sure.
Good luck...