Classic early symptoms of a magnesium deficiency. Good eye, you caught it at a good time.
Make a solution of 1/2 teaspoon Epsom salts (MgSO4) in a gallon of water. Use this as a foliar feed.
To prevent the problem from recurring, get your soil pH down to the 6.5-6.8 range.
Also, avoid overwatering. I can't really see the soil, but it looks unusually dark- check to make sure it is not soggy.
Your next watering, adjust the pH to 6.5 (use cider vinegar if you have no other choice), which will start to lower the soil pH a bit.
Micros get locked out above 6.8 and below 6.5.