Dettox mould and mildew remover got rid of stains without any scrubbing/effort on some of the lining inside our boat which had surface mildew before we re-painted. It will probably do the same for sails. I would really rinse it and not leave it on for long because it contains bleach and may damage or weaken the fabric if left too long.
I'm sure that some time back someone suggested Biotex washing powder. In fact if i remember rightly Sailbad was going to re-package and sell for a ridiculous mark-up!
I've used a product called "cinch" on mildew stains, which really works well. haven't tried it on sails though. might try some on a smal spot or spare
piece of sailcloth.