MUSTY ODOR: There are two common causes for a musty odor of the pool water:
TOO LITTLE CHLORINE: The discussion on Chlorine Demand stated that chlorine must kill all bacteria, living organisms, ammonia, and other contaminants in the water. Well, chlorine is very effective at killing bacteria, living organisms, and other contaminates (which again are dirt, debris and algae spores), but when ammonia is present, chlorine will combine with it, but cannot successfully kill it, and the two combine to form "Chloramines." Chloramines will remain in the water, and will register as chlorine, but are ineffective at sanitizing, disinfecting, or oxidizing. Although Chloramines register as chlorine, they are undesired. Since this chlorine is now virtually useless, you technically have too little chlorine in the pool. Having such a low level of chlorine in the water may cause a musty odor.
So then, since chloramines are ineffective and undesired forms of chlorine, how are they removed from the water? Easy - by shocking the pool on a consistent and
regular basis. Even if you maintain a 2.0 - 3.0 ppm (ideal) chlorine reading at all times by merely adding chlorine tablets or granular chlorine to the pool, the presence of ammonia will lead to Chloramines, preventing the chlorine from being fully effective. You must shock the pool.
THE PRESENCE OF ALGAE: A musty odor will accompany the presence of algae. Although foul, the odor will be the least of your worries. Your main concern will be treating the algae and restoring water clarity.