A green swimming pool is caused by green algae, the most common type of algae to plague pool owners. Other pool problems are mustard algae—it looks like yellowish or brownish slime on the sides and bottom. A little rarer is black algae, which is the toughest to eliminate. They will coat the wall, and require scrubbing with a brush to remove their outer protective layer. One of the most common ways algae may be introduced to your pool is through the landscaping. Beautiful, lush landscaping close to the pool will create conditions for algae to take a foothold. You may want to keep plant matter like leaves trimmed back and cleaned off the deck so they don’t blow into the pool. Heavy rains and high temperatures can also spur algae growth.