What is this on my seedling?

Hey guys,
This happened from night to day. It is now full of brown spots. Can anyone help me identify the problem?

People are going to want some info pertinent to your grow. Hard to help without it

The plant is 3 weeks old... No nutrients used so far, only CalMag and the environment is at 70% RH plus 25ºC. I have a total of 1000w and they,re close to the plant as the makers specs tells. The ph is at 6.2ish always but I sure float a bit.