It is whatever the material lining the mirror box is, that is coming off. I highlighted it in red in this picture. I rotated this image so you are now looking straight on at the camera. The silver ring is the lens mount, the highlighted area is the side of the mirror box and the right hand side is the actual mirror.
From what I can tell there does not appear to be any contact between the mirror and the material.
Have you used any non standard lens or adaptor for mounting another lens............as the position and 'mirror image' of damage looks it could have been caused by something rotated into the mount with a protruberance striking the mirror box lining. Such rotation & return makes me think of mounting a lens!