that sure looks like a mold or fungus to me. Does it flake off? The brown spots one, looks like a mold that would flake off to me. If that's the case then I would be leaning towards humidity being the issue. If that is mold I would think you want to remove that from your grow space immediately.

The other plants look great. That sure is puzzling, when one does something and the others don't. Be careful it doesn't spread.