I may be wrong but I'm thinking you've got an issue with some spider mites... or aphids... I don't think this is a nute problem at all. If this was my grow, I'd get out my Dr. Zymes Eliminator - it comes in concentrated form, is organic, can be used up to the day of harvest and will kill and eliminate "mites, thrips, aphids, whiteflies" - both adults and larvae/eggs... it's my "go-to" when I don't know exactly what kind of pest I'm dealing with although it won't do much if anything for leaf munchers... THOSE I use something else entirely. But the underneath side of your leaf suggest Dr. Zymes will take care of the problem. Use it at light's out...
Good luck!