Luckily, my pool's cya drops over the winter and the pool is crystal clear and not nasty. I haven't "closed/winterized" my pool for the last 2 winters and luckily, have not had any problems, knock on wood. However, I have been battling a very small amt of mustard algae this season for the first time, but my CYA seems to go to zero over the winter every year, whether I open to a clear pool, or keep it open and clear all year. I had it tested for nitrates which I believe Ben said could cause that drop, but there were no nitrates, according to the pool store. I believe that test was sent out for testing, and not done at the store, but that was a good few years ago.
John