The black and green stuff embedded in your walkways is mold. Mold can cause health issues if it is allowed to grow unchecked. It's important that you keep your concrete and outdoor areas clean so that mold doesn't have a place to grow. Some of the most common places mold is found include:
Sidewalks, driveways, and patios
Porches
Walkways along the side of your home
Be sure to reach out to an expert if keeping these areas clean becomes a hassle.