I saw this today on one young plant. I have seen it before on others. I moved LED to 18” away. Was closer by 4”. Using on light as plants are only 6” tall. I have cal-mag but have not applied it so far. I know a lot of LED light can cause issues. Cal mag is one solution for an LED problem. Looks coppery under the LED. This was photo’s outside tent. Very little nutes used yet. Thanks