The fungi on roofs lack chlorophyll. They are unable to manufacture food from raw materials, so they must obtain nutrition from some form of organic matter. This means that like algae, they need a warm, humid environment to thrive.
Another thing that fungi has in common with algae is that it, too consumes asphalt. It also consumes the ceramic granules that protect against damaging UV radiation and insulate the roof against extreme heat (another reason the roof overheats).
Not all mold (fungi) and algae pose a danger to health. However, some do, especially for those who suffer from allergies, asthma, and respiratory problems. Fungi and algae spores can get inside your home because they end up on driveways and sidewalks from rain, and are easily tracked indoors. They can even be drawn into your air conditioning systems and populate the ducts.
Different mold species can have different effects, but it is very important to remember that any excessive mold growth should be taken care of.