I went through a period of that as well. It's usually not a problem if EVERYTHING else is good. I suspect it was that some algae forms inside my drop-in steps that not enough chlorine can get to. Last year, I put a floater with some pucks INSIDE the steps shut down to it's lowest level. That helped for most of the season.

This year I'm trying to use 80# bags of water softener salt to hold the steps down and they aren't perfectly sealed, so some saline is leaching into the pool with the highest concentration under the steps. So far, no CC....