Mild soap and water. Powerwasher on low setting. You can also visit your local big box store, they will have some chemicals designed to remove algae.
You don't want to use too harsh of a chemical or you may stain/bleach the siding.
I've used a product call "Mildew Check" and seems to work well and leave a protective coating behind to prevent reoccurring algae growth.
You can power wash it with detergent which is made specifically for vinyl siding. Use a low pressure and try to wash the siding head on as this will decrease the amount of water which goes behind the siding