Unfortunately, I can't return it, from looking at it without the mirror is pristine. I see a what appears to be the detached spot on the hazy white glass that is above the mirror, but I was unable to see anything that would have caused the bottom part. Does this sound like it would cause any problems to the photos, because if not I am fine with keeping it the way it is.
What happens is they use a foam seal where the pentaprism joints are. The foam deteriorates and eats the silver off the mirrors in the prism.
This issue does not effect what is recorded on the film but can make composing a bit annoying. I guess you can get used to it.
I'd inspect the back foam seals and also the mirror bumper foam as those have probably gone south also.