I would wait to see if it spreads. The worst thing you can do is to panic over everything you see, and make a small problem much worse.
Check all your leaves over and under) with a magnifying glass to rule out any infestation. If you don't see anything, then just leave it for a few days and monitor to see if it spreads. Otherwise it looks quite health.
I have the same problem, it will start spreading out.
Got an answer of a fellow grower on this forum; think it will be K (Potassium)-deficiency.
I started adding more Bloom-nutrients.