If you look up at homes across the Peninsula, you’ll frequently spot dark, vertical stains running down the shingles. Many homeowners assume it’s soot from a nearby freeway, dirt, or simple weathering. In reality, those black stains are a hardy cyanobacteria called Gloeocapsa magma (blue-green algae). Thanks to the persistent, heavy coastal fog rolling through Daly City and the surrounding hills, our neighborhoods provide the exact high-humidity, low-sunlight environment this organism needs to thrive. Leaving it to grow unchecked can...