I'd try vinegar and water and see if that works, you could also make a paste of baking soda and a little vinegar and let that sit on it for a few minutes. It almost looks burnt to me. Other members will have more advice for you.

Pans with aluminum, copper, or cast iron bottoms can cause stains on some glass tops. Try the vinegar/baking soda Christine 5b suggested.

Thanks for the responses! We haven't been using aluminum, copper, or cast iron pans. I did some research online and found that Bar Keepers Friend works really well? I ordered some from Amazon and will post my results!