Even a well-sanitized pool can fall victim to algae if phosphates are high or circulation is poor. Using a preventative algaecide once a week acts as an insurance policy against green, mustard, or black algae blooms. Additionally, check your cyanuric acid levels. Often called "sunscreen for chlorine," cyanuric acid protects your chlorine from evaporating under sunlight, though too much can make your sanitizer ineffective.
Free chlorine, pH, total alkalinity, and calcium hardness. pool chemical maintenance