Readings took a turn today - FC holds but CYA is gone

[Watermom]

As long as you only lose 1ppm of chlorine overnight, that is fine.

Combined chlorine is what forms when free chlorine combines with ammonia, nitrogen, and other "pollution" in your water and loses its ability to sanitize the water and kill viruses and bacteria. (A lot of the "pollution" is from sweat, urine, algae, etc.) It is typically what you are smelling when "a pool smells like chlorine." You are smelling the combined chlorine. Ideally, you want the CC (combined chlorine) level to be 0, but up to 0.5 is generally not a problem. The way to rid the pool of CC is to shock it. That is not a very technical explanation, but hopefully it will help clarify it for you somewhat.