That's from you getting water on the leaves under led it burns them try not to get the leafs wet and you shouldn't have burn also keep you light far away until there much larger Sent from my XT1585 using Tapatalk
If you post pics, please take NOT under LEDs. Can't tell by this. Stop foluar feeding now. They get fed by soil/media at this point.
I'd say 3-4feet away from the seedlings until the first set of true leads show and gradually move closer not all at once give it time to adjust Sent from my XT1585 using Tapatalk
Water drops on the leaves can focus the light like a magnifying glass leaving burn spots. Spray at lights out.