Fungus damage can be removed by using cerium oxide mixed with water as a polish. People make telescope mirrors all the time and the final step is polishing. Check the web for suppliers. As mentioned above the coating will have to be re-applied. When I was in graduate school I had to polish sodium chloride optical windows all the time. Only difference was using ethyl alcohol instead of water. (Sodium chloride transmits infra-red light while glass will not.)