I would call magnesium deficiency. The interveinal chlorosis is a Mg def sign.
Also the red stems on leaves is often a sign of magnesium deficiency also. Phosphorus deficiency can turn the stems red too.
The darker spots there could be just further progressed Mg deficiency.
Since it's the only plant showing this, check if it is the closest to the light? Could be just getting bit more light than it can handle, since it's progressing from down up, instead of burning the very top leaves.
Other than that depends are the rest of the plants same clones or different strains?
Could be just more hungry than the other ladies.
The above point about calcium, magnesium and potassium is a valid point. Too much of any of them, reduces the amount it gets the other two.
So overdoing calcium and potassium could lead to magnesium deficiency, even if there is enough magnesium in the soil.