40 grip sandpaper strong beach some battery acid and a power washer should be enough to clean it if you find that you can see the moon and stars after my advice you got the mixture wrong be carful ? soap and water nothing else or you will do damage to seals just soap and water be carful
I use Fenwicks Motorhome cleaner on a DRY roof. Spray on a smallish area. Leave for a minute or two. Rub it around with a long handled brush. Rinse off. Start at the lowest point of the roof.
I often finish with Demon Shine so the roof stays cleaner for longer.