Concrete algae is dangerous and unhealthy!
If left untreated, the algae will permanently discolor the cement or concrete. In addition, algae growth is rather slippery and will become a safety hazard when it is allowed to grow on walkways. https://www.hunker.com/13420688/how-to-get-rid-of-green-algae-on-cement-or-concrete#:~:text=If%20left%20untreated%2C%20the%20algae,allowed%20to%20grow%20on%20walkways.