Your shock levels of 12-15 are a little low-that's for people with lower CYA levels than yours. With a CYA of 50, you should shock it to a minimum of 15 ppm, and since you're at the upper end of the range, I would go between 15-20. Check out the "best guess chlorine chart" in the link in Watermom's sig for the chart that explains the CYA/chlorine level relationship.

You probably do need a higher CYA level with your SWCG, as Watermom indicated, but until you get the pool cleared up, I wouldn't add more, and would just shut the SWCG off and clear it with bleach. No need in wasting cell life!