It looks like Nitrogen (N)- "Deficient plants will exhibit uniform light green to yellow on older leaves, these leaves may die and drop". Add some NUTRIENT with more Nitrogen and it should be ok.
OR... u have a nutrient lockout. Maybe caused by low pH. What is the runoff pH ?
Strain: Feminised Automatic White Moscow
40-45 days from seed
first 2 weeks 20/4
after the first 2 weeks 12/12 - I know that this kind ot strain have to be 20/4 or 18/6 during the whole plant's life but I had another plant in the box that already was at 12/12 so I put the little Moscow in...