What is this and how can I fix it?

Just by the leafs looks like a magnesium deficiency. But the whole plant must be taken into account...

That is a magnesium deficiency, i see the tip of this leave pointing down, which suggests watering issue. But... Hard to tell w/o seeing whole plant and situation. So could be too much water, could be not feeding calmag while growing under LED, could be PH that is too high or low, preventing this. Let soil dry, do a foliar spray with some calmag.

Yeah, calmag, imo.

Looks like a cal mag deficiency to me.