The film is from moisture getting into your watch. Moisture can get in one of 3 ways generally. The crystal if cracked, the crown, or the back. In your case since you have buttons for the mulch-functions of the watch you add a possible place for moisture to enter. Take the watch to a watchmaker or watch repair person. Your watch has to be taken out of the case and the case needs to be cleaned. Next the watch tec. may be able to see just where the seals failed and replace them. We of course do this. Also if the gasket is old or out of shape this is a culprit as well.
Hope this helps, gabe