Magnesium deficiency.
You can't do much to fix a deficient plant in such a tiny pot- the soil conditions fluctuate more wildly and you could be providing ample Mg but the plant may not have the conditions to uptake it due to moisture, pH, oxygen problems in the soil.
That plant needs a gallon pot of LIGHT soil. Read the sticky in my sig. You may foliar feed with a weak solution of Epsom salts (1/2 teaspoon per gallon of warm water) to keep that deficiency from getting worse until the soil conditions are changed.