If it's recommended by your fp manufacturer, use a fp glass cleaner (one for ceramic glass). We got a bottle of a glass cleaning "cream" at a fp showroom. I use it on the glass (in/out) and on the shiny black enamel panels lining the side and back walls of our gas fp insert.

You definitely want to clean that residue off the glass before it really bakes onto it and becomes even more difficult to remove.

We've only had our insert for two years, but I plan to clean the glass, walls, and the glass crystals on the floor of the insert, every summer/fall. This way it's nice and clean, and ready to use when the weather gets colder.

It's residue moisture. We just saw it for the first time on our gas unit this year. I kept the pilot on this summer for then first time. It is a result off condensation forming when very humid. Windex will take care of it. Be careful removing the glass to clean the inside. Robax glass is very expensive, I broke mine cleaning it last year.

It's my understanding that you should avoid using glass cleaners that contain ammonia on fp glass.

I was told it was OK, have used Windex on the glass for 13 years. Perhaps it depends on what type of glass, ours is Robax.