No! It may be low maintenance, but anything left outside in the humidity and sun will get dirty.
Most likely it is mold. Mold can grow anywhere there is humidity and a food source.
Yes! Especially for those who have asthma and allergies. We can remove mold from most surfaces.
If they are in areas near or above mulch most likely they are Artillery Fungus spores. This fungus lives in rotting hardwood mulch and shoots its spores in the air.

No, you do not need to be home. Everything we do is done outside the home.