You may have what we call mustard algae in your pool. Mustard algae is a form of algae that often is present on pool underwater surfaces with a yellow or off-white tinge. When brushed off a pool surface it will create a yellow and green cloud in the water. Unfortunately, yellow algae is EXTREMELY resistant to chlorine, meaning it can still be introduced and grow in pool water that is optimized and properly sanitized for swimming and pool equipment.
The optimal free-chlorine sanitizer level for a residential swimming pool is 1-3 ppm of free chlorine. Any higher free-chlorine in pool water, runs the risk of having too much chlorine, which can very detrimental to the pool swimmer (eyes, ears, nose) and pool equipment. Even when keeping the pool filter adequately clean, free-chlorine levels must be kept around 7 ppm or higher in summer months to fully eliminate any potential for mustard algae growth. This level is WELL higher than the recommended safe levels for ideal swimming conditions.
Unfortunately, in order to properly maintain the pool and ideal swimming conditions, pool care companies are forced to run the risk that these algae may be introduced to the pool and grow. When they are found, the mustard algae must be killed with a thorough pool brushing, filter clean, and pool “shocking” (essentially adding a large amount of chlorine to raise the free-chlorine above 8 ppm and kill the mustard algae).