Magnesium deficiency or lockout.
Foliar feed with 1/4 teaspoon Epsom salts per quart of water with a drop liquid dish soap as a spreader-sticker.
The other problem is just a droplet of fert solution that has evaporated and left the salts behind in the leaf.
The fact you see so much salt in that droplet residue makes me think you are overfeeding to the point of lockout... fill out the form and we shall see.