The seal between the filler and the glass can develop a small hole allowing air into the enclosed space between the glass panes. The moist air starts out looking like a slight shading between the glass. This continues to deteriorate until there is visible moisture. The moisture slowly morphs into a whitish film on the glass which blocks visibility. It cannot be wiped off because it’s between the glass.
There is no need to replace the sash, nearly all sashes have removable glass units that can be removed from the sash. Replacement glass can be ordered or made by a local company and easily installed.
It’s a failed window glass seal, easily replaced and cost effective repair to an annoying problem.