I just looked at mine, and I have something similar. It doesn't look as big as yours, but it is in about the same place. It looks to me like a slight glue or paint separation on the bottom side of the viewfinder prism. Unless it gets bigger or moves around, I wouldn't worry about it. I doubt it has any affect on viewing as that spot is well masked when I put my eye up to the finder.
A few loose screws short of a functioning camera.