There may be film-like substances floating in hot water, or rust-like spots (scale) or rough substances adhering to the inner surface of the inner container. These are caused by the components of the water (minerals, metal ions, etc.) rather than discoloration or corrosion of the inner container itself. It may cause the humidifier to make a louder boiling sound or produce an odor, so please clean it regularly (every 1 to 2 months) with citric acid.