Light Green Spots on leaves

Looks like a calcium deficiency is starting. If there is any coco in medium a cal mag is needed during the grow. Good luck and happy growing 🌱

I am using Promix HP as a medium. Upon further inspection the colloidal leaves on one plant are yellow and starting to see it on the lower true leaves

possibly start of some interveinal chlorosis.. a few things can cause this. it's impossible to know what without more information or to allow symptoms to progress and still more info needed. you'd have to calculate what you are providing and if soil, how hot the substrate is. I'd see how those newest leaves look in a day or two... if no spots i'd chalk it up to ugly early leaves. is that coco? soilless? feed a 650-750ppm well-balanced diet everytime you irrigate. as long as you get 10-15% runoff and don't dump an entire gallon or two on top all at once, it'll ensure stable pH and concentration of nutrients in substrate. so, if underfeeding, definitely step up to a healthy range. If you want ballpark ppm of NPK et al, i have a table in nearly any week of nearly all but my oldest diaries.. take a pick. Ca, Mg, S will be impacted by your tap water. i have hard water at 250-300ppm. i probably get more of Mg/Ca from my tap than most, but the values will still be fairly close. if soilless, you can irrigate safely at 650-750ppm right fromstart... a seedling won't make use of it, but it's not "hot" by any means, so ramping up first couple weeks may save a few pennies? This concenration level merely keeps up with metabolic rate and mimics / based upon the most fertile lands in world. 120s / 50s / 190s ish... npk, respectively. american standard labelswould come out to a roughly 2:1:2 ratio, i believe. an odd plant may say differently, but most will be happy.