I live in the same region and also use a mesh cover over a vinyl liner. I, too, add lots of PolyQuat, then shock the water up to 15ppm prior to closing.

It has been unseasonably warm, but we have had some sub-freezing too. It doesn't hurt to check but check your water temp as well. If the water is well below 60, you shouldn't be seeing a problem. If it's below 50, better still.

If you decide to add bleach, I would strongly suggest that you dribble it around the edges--don't just add it in one spot or you can damage the liner. Because of the circulation being, well, non-existent, I'd stick to bleach, not the stronger stuff. If FC is low, you may want to consider buying or renting a submersible sump pump to get the water moving.