Looks like you have an algae problem. With a CYA of 20, you will need at least a FC of 12 ppm to kill the algae but better to start at 24 ppm since it will be used up fairly quickly killing algae and converting your high combined chlorine. For 12% chlorine, 24 ppm is 2 gallons of chlorine per 10000 gallons of pool water. For 6% you will need twice as much. Keep the CL level to at least 12 ppm until the algae is gone and the pool is clear. Once you kill the algae, the pool may turn cloudy. This is good. Keep a watch on the chlorine so it does not drop during this period. Also, keep the pump going 24/7 and vacuum the dead algae up when you see it on the bottom. Good luck.