I recently bought my friend's Minolta XG-1 which looks super well maintained cosmetically. He did mention to me that it was sitting in his hot attic for about a year.
I noticed that there is a weird black line covering about 25% of the viewfinder. It has a weird bubbly texture and I suspect it has to do with something on the prism itself. I tried cleaning and thoroughly inspecting the mirror and the focusing screen (I am new to photography so I am not sure but I think that's what the thing above the mirror is called) and found nothing suspicious. There is barely a single speck of dust left after cleaning everything.
Additionally, the weird line in the viewfinder is still present if I look through it without a lens attached.
What steps could I take to attempt to clean this out?
This is a picture of what I see through the viewfinder: