You've got the early stages of a micronute deficiency there, specifically Mg.

You may very well be overwatering- does that pot have a drip tray built in? Those drain poorly, and if so, you should not only remove the drip tray, but try to drill a few extra holes even in the SIDES of the pot.

Then, what is the runoff pH of the water coming out? What is the pH that you are putting in? What type of medium (soil or soilless)?

Next, treat the deficiency. A spray bottle (about a quart) with a generous pinch (about 1/4 teaspoon) of Epsom salts will treat the symptoms of Mg deficiency.
Nice catch, that's good you noticed it early.