Replace
Fog or haze between the panes: Replace the glass unit or the window
If the haze is inside the glass, it will not wipe off from either side. It means the seal around the double-pane unit has failed, and the gas that insulated it has leaked out.
Some glass shops can swap just the sealed unit. On an older window, a new window usually makes more sense than new glass in a tired frame.
Before anything else, look for the maker’s name on the frame: many window warranties cover a failed seal.