The answer to 'why is my flashing algae growth' is that algae growth on roof flashing is a common issue caused by moisture, shade, and organic debris buildup. Flashing is the metal or plastic material used to seal roof joints and transitions, and it can provide an ideal environment for algae to thrive.
Symptoms of algae growth on flashing include a green, slimy, or discolored appearance. This can be more than just an aesthetic issue, as the algae can trap moisture and lead to further deterioration of the flashing and roofing materials over time. Proper cleaning and maintenance is important to prevent this problem.