Why is this seedling looking like this all my others look healthy ?

She is very close to beeing dead, by colour looks like she is in huge trouble. I doubt she will make it, or you will nurse her two weeks while she will return ... Better start new. That necrosis and such yellowish pale worries me a lot, your soil I humid too, but I never seen anything such rapid and only to one and I had many mistakes ... Mold, mildew, fungus ??? Good luck !