Main cause is greasy hands touching the glass. Don't touch the glass. But if you are talking about algae then avoid direct sunlight exposure and run an UV sterilizer.
Yeah sorry should have been more specific, i'm talking about algae, the brown and the "green spot".
Have started keeping the blind fully closed at that end of the room and as coincidence would have it, i've recently started running a UV too.
Cheers, just wanted to check i was doing all I could.