green spots

First it looks like moisture stress from leaves resting on each other at some point. If you know it's not moisture then- Looks like it isn't on the lowest set of leaves, so its probably a deficiency. Everything else still looks healthy, start by checking your pH. If your pH is in the proper zone 6.2-6.5 then its probably just needing more Cal-mag. From what I noticed on my plants is they start out with little markings just like that and a day or two later they start turning a yellow or reddish color. For me giving my plants more cal-mag always fixed this problem. I'd start adding a little more cal-mag to see if that fixes the problem from continuing. It's hard to feed a plant to much cal-mag but still don't over do it. After a week it should stop or even sooner depending on how heavily that plant feeds. The markings will never go away, but once you stop them growing or spreading then you know the plant is back to 100%