compur
Been spending a lot of time on here!
I've never worked on a Flexaret so I don't know about them but the mirrors on some low cost TLRs were made of metal (instead of glass) that was chrome plated or something to give it a mirror-like finish. In time the finish on the metal would discolor and turn a reddish (rusty) color due to oxidation.
That could be what is happening here. In that case cleaning the mirror with a chrome cleaner should fix it.
That could be what is happening here. In that case cleaning the mirror with a chrome cleaner should fix it.