An inert gas is placed between the glass for insulation. If it is not insulated enough, it can mix with the moist ambient air. At low temperatures, the glass falls off.
This has happened several times in our rental apartment. Unlike windows with no moisture inside, the affected windows were much colder during the colder months.
This was reported to the landlord as a defect and replaced each time...
My guess is a tiny leak in the window pane seal. The gas then exchanges with the surrounding air with all of its components. I don't think it will go away on its own.