Gas fireplace glass may appear cloudy for two reasons. Firstly, and most commonly, it is caused when sulfur released from the gas mixes with moisture, staining the glass with sulfuric acid. This must be removed regularly to prevent permanent damage. Alternatively, if you cannot wipe the cloudiness away, it has likely been scratched during the cleaning process. This is commonly caused by using too abrasive of a cleaning product or scrubbing too harshly when trying to remove stains.