you can buy path cleaners its quite cheap at wilkinsons..and also if you get a hard brush some boiling water and jeyes fluid you can scrub the green off...and the jeyes fluid is strong and will kill the green stuff..or you can buy ..hire..or borrow a power washer..
I use diluted bleach in a bucket of water. Pour it over the paving slabs & brush with a stiff yard broom. Then hose it down - lovely and clean. I have tried various 'specialist' cleaners, including jeyes fluid, but none of them seemed to work too well so thats why I opted for the bleach treatment. hope this helps.