Does that look like an early deficiency?

You'd have to see how it progresses to eliminat possibilities. If you feel the plant is still a bit overfed otehrwise, you can withold some and add what is deficient, no big deal. Faint sings of interveinal chlorosis. Starting lower on the plant? at least the leaf in the pictuers looks to be fairly low on the plant. Even too much of something else can cause lockout of another molecule. I don't think this is Mg-def, so i'd lean toward something being locked out, especially if you are in the midst of trying to dilute the EC of the substrate. if you start seeing spots with the chlorosis, then Mg comes into play. if your nute products don't have enough, you can add a little epsom salt (any generic unscented foodgrade works fine - don't pay for 'horticultural' epsom because it's the exact same thing atom for atom).