You may be able to buy the CYA at Lowe's, Home Depot, or Walmart. It may be labeled as stabilizer or conditioner. Check the label. If the ingredients say cyanuric or isocynuric acid, it is the right stuff. If those place don't have it, your pool store will.

If you consistently sustain the high cl reading, it will kill the algae. But, you have to be diligent about it and stay on top of it. If you take a day off, then you lose ground. Brushing your pool will speed the process.