Algae can form in a swimming pool if the water is not properly maintained. It relies on sunlight and the oxygen in the pool water for photosynthesis and it can quickly spread across the lining of an afflicted swimming pool. In addition to negatively affecting your pool's appearance, if algae is able to grow in the water other organisms could be forming in the pool as well. Even though black algae is simple to spot, removing if from a pool with a vinyl liner requires care and diligence.
Scrub the vinyl liner with a nylon brush to help dislodge any algae that has attached to the liner. Don't use a metal brush, which could tear the vinyl material in the liner.
Shock the pool water. Pool shock is a hyper-chlorinated solution that helps kill off algae and other organisms in the water. Follow all instructions on the bottle in regard to how to add the shock to the water and how much to use. Brush the pool again once the shock has been added.
Apply a polymer algaecide to the water as well, which will target the black algae specifically. Continue brushing the pool at least twice a day until all the algae is gone.
Run the pool filter continuously until all the algae is removed.