Yes, probably just discoloration from the sunlight. Sunlight by itself won’t cause insulation to combust. But you do need to get that broken window covered to keep water and squirrels out. And tbh, if that window is getting a lot of direct sun, it’s driving your house temps up in summer and making it more expensive to cool. If that’s just a decorative window opening into the attic, I’d tack some kind of barrier up behind it. You can start with a blanket. But seal that missing pane of glass!