That green gunk is algae growth, and you'll get it in areas of the house that don't get much sun to keep the siding dry. We get it too, on the front of the house which only gets the early morning sun.

If you have a pressure washer, just the plain water will take it off without adding any cleaners or chemicals. That's what my son does. If you don't use a cleaner that has a growth inhibitor, it will come back every year and have to be done again, but at least you won't be putting any chemicals in the ground for your pets to get into.