If your pool water has altered from very low to high pH levels in a short space of time, this may result in dissolved metals coming out of solution and accumulating on your pool surfaces. This issue is also heightened by high calcium levels in your pool from the use of granular calcium-based chlorine products. You should purchase a metal remover to assist, although severe cases may require draining your pool and acid washing your walls. Alternatively, you could add hydrochloric acid until the metals redissolve. Your metal remover can then be added after either draining your pool or adding acid, following which you should raise the pH of your pool again.