Most likely it was used up if you have green stuff present. To clear up the green stuff, you need to get your chlorine up to the 12-15 ppm range and hold it there until it clears, filter running.In a 36K gallon pool, each gallon of 6% bleach should raise your FC by 1.7 ppm, so you can use that as a guide when testing and adding more bleach to get back up to the shock level.

Janet