I dont nute till flower something that many people have different opinions about, but Im like Cereal Box and have only grown in soil so I am out of my league here but to take an adventurous guess you probably have some sort of nutrient deficiency
Yeah, your best bet is to take this to the hydro section. It looks like a magnesium def to me but I honestly couldn't tell you how much to feed with. The guys in the hydro section will know and help you out. Good luck!