Since no one has replied I will take a shot in the dark. You said it looks like baked on salt creep/milky white smears, are you sure it's not on the outside glass? The reason I ask is if you put your hands in the tank and get water drip on the glass then wipe the glass with a towel or similar with out some type of glass cleaner it will put a milky smear on the glass. If you come back later with a glass cleaner and clean the glass it will not come off with one or two cleaning. Do you clean your glass Vertically? may be the cause but like I said shot in the dark.
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