Yellow/brown marks

It does look like they have had a bit of burn from some water droplets. are you balancing out the waters ph as well? or just adding in calmg to its water? Your diary says the lamps distance to the plants is 10cm, or just under 4 inches. That is way to close. If it's meant to be 10 inches that's also a little close for seedlings sometimes. I would aim for anywhere between 25-35" for sprouts on a 600w led if that the true wattage its pulling. Some of them also have dimmers. Some of this could be light stress but I'd like to confirm the lighting distance before thinking about that. If you light manufacture has a PPFD map you can reference then use it as a guide line and adjust from there. Good Luck and Happy Growing!