I'm going to move this to problems, since you DO have a problem.
That plant is showing classic but mild Mg deficiency. You need to identify the cause, but in the meantime, dissolve 1/2 teaspoon of Epsom salts in a gallon of tepid water and mist the plant lightly with that. Only do this a couple times.

Be sure that your soil pH is 6.5-6.9 and your soil is free-draining and not sitting on a cold surface.

That plant is old enough to start feeding a weak solution of a complete grow fert. The first week, only feed at 1/4-1/2 the recommended strength.