For anything outdoors, I recommend a product called Jomax. It's a house cleaner and mildew killer that effectively cleans any exterior surface, from siding to tiles, and won't harm shrubbery or lawns.
Another option is TSP - trisodium phosphate. It's available in the painting aisle of most hardware stores and home centers. TSP can be mixed into a soapy solution, brushed onto any outdoor surface, scrubbed and rinsed away with good results. A heads up, though, that it can have a bleaching effect.
Whether you go with Jomax or TSP, I suggest leaving the solution on for a few minutes before rinsing it off for best results.