A couple things can cause the white film. At times if you use a window cleaner to get debris or oils on your glass, it may leave a residue which won’t show up until you fire.
Another likely cause is when thin fire shelf paper is used, if your not ventilating the kiln when the paper starts smoking, it will get in between the glass and leave a smoky white film.
Im sure one of these will be the cause! Let me know.