How To Clean Cloudy Fish Tank?

i have a 10 gallon fish tank that is not in use yet. its 1 foot tall and about 4 inches down from the top its all foggy and cloudy. its only like this when its dry but when i wet it, it becomes clear but then dries up quickly and becomes cloudy again. ive put water in it 4 times and let it sit there for a while but is still cloudy. what can i do to stop the cloudiness?