I think that I have some instructions on my PC. Went through a phase of collecting Nikon repair manuals & stuff & I think that I have some instructions for taking it apart & cleaning the resistor. Sadly I am not going to get near my PC until late tomorrow night.
I have had mine apart. The screws to take the top cover off are under the leatherette. There are 4 of them IIRC. By now expect the leatherette to be very brittle so you will need care peeling it off.
Are you sure it is mould & isn't the mirror de-silvering? This is what happened to mine, right along the upper line of the prism. The meter was also not working on mine so in the end I wound up buying an eye level finder which cost as much again as I had paid for the body with the FTN finder. Lovely camera to use though.
I will check the thread again tomorrow night & if you haven't had anything, I will see what I can find.