A. The inside surface of the watch glass may fog or mist when the watch is exposed to a sudden drop in temperature. No problem is indicated if the fogging clears up relatively quickly. Sudden and extreme temperature changes (such as coming into an air-conditioned room in the summer and standing close to an air conditioner outlet, or leaving a heated room in the winter and allowing your watch to come into contact with snow) can cause it to take longer for glass fogging to clear up. If glass fogging does not clear up or if you notice moisture inside of the glass, immediately stop using the watch and send it to our Service Centre immediately for a repair and/or assessment.
https://casiouksupport.zendesk.com/hc/en-gb/p/ServicingAndRepairs