The answer to 'why is my fascia algae growth' is that the fascia board on your roof is likely experiencing algae buildup due to moisture, lack of sunlight, and poor ventilation. Fascia boards, which are the vertical boards that run along the edge of your roof, can be prone to algae growth in damp, shaded areas where moisture accumulates.
Algae growth on the fascia is a common issue, especially for older homes or those with overhanging trees. The algae can appear as a green, slimy buildup and can spread over time if left untreated. This can lead to deterioration of the fascia board and even structural damage if the problem is severe.