You can buy 'Patio cleaners' of various sorts, and we've tried them all! Some you brush on and leave, some you just water on, and brush off, and you can buy them on line or from your local DIY shop. There's a website here which helps you find information: http://www.safegardening.co.uk/PatioPathCleaner.html
We found nothing works properly except power washing, and that makes such an amazing difference - you can write patterns in the algae as you're washing it off. I do ours once a year now, and it stays reasonably clean and clear of algae until about Christmas time, and a long damp period seems to turn it green again. This year it's not too bad because it's been too cold.