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.