Use Algaecides to prevent algae growth in swimming pools and kill existing algae that survive general chlorine dosages. Algae is an aquatic plant that grows rapidly in swimming pool and spa water when a combination of warm water and lack of chlorine or other sanitizing agents exist. The most common types of swimming pool water algae are green, black, mustard, and pink. Initial signs of algae growth are green or cloudy water, slipperiness on pool surface, and spots on pool floor or walls.