Black and Green Algae are commonly misnamed as "Mildew or Molds" or even "Fungus". Algae is naturally in the air. And grows on surfaces where it finds a food source (example: Limestone in shingle, Moisture on roof or gutter line from shade).
Both Black and Green Algae causes shingle granule loss by "eating" at the shingle loosening the granular. Also keeping moisture on the roof. As well as being an eyesore.