I have the same problem . I use a product called "Pro-Clean" For patios decking etc. find it on line on a well known auction site .It can also be used on cars ,caravans garden furniture . Its brilliant . . It can be bought ready touse or concentrated and diluted up to 5 to 1 ith water. Ive used it on two of my cars and it works well doesnt damge or stain paint . and my friend used it on her audis soft top that as going green and it worked brilliantly . Just spray it on in the evening wetting all the mould and algea . leave for a few days and then wash the car . On patios it can take a few eeks on really engrained mould but suddenly after heavy rain stoneork looks like new again . Green mould and algea will be gone and it prevents re growth for a few months .
Just to add I have nothing to do with the company just a happy customer . you can probably tell I,m well pleased with this product !. Hope it helps