Jim,
Obviously you want to try to figure out what the fog is...can you scrape it with a razor blade? Glass is glass so you have to be careful but you can use some abrasive type cleaners and not hurt it. I use brass wool pads - not steel wool, those will scratch the glass - try the brass wool and some Clorox Soft Scrub. You can get different grades of brass wool ( 0,00,000 )from marine supply store or order from McMaster Carr.
If that does not work Eastwood Automotive products has a glass polishing powder/kit that will for take it off. It takes a lot of time and patience. Make sure you support the glass directly under where you are working, do not let it just lay on the ends when applying pressure. I know this stuff works because I used it on some rare automotive glass during a restoration.
Tom.