m0useanswered grow question a year ago I agree its magnesium, I don't see any spotting suggesting calcium is lacking but its a common component of magnesium supplements for cannabis. calcium and magnesium aka calmg or CaMg.
If you know the calcium is fine in the medium no sense adding more, you can use common bath salts magnesium sulphate to give it some extra Mg. Little goes a long way. 1 teaspoon per gal of water. to start then go from there.
Good Luck!