Yes, the lower temperatures mean algae growth is low or non-existent. Not having bather load means you aren't adding ammonia compounds nor organics into the pool (except for what blows in). And this time of year the sun is lower in the sky and not as strong (about 60%, depending on latitude; cloudy days have much less UV) so less chlorine gets broken down by sunlight. Put all this together, and the chlorine usage goes way, way down. If you add a cover to your pool, the chlorine may only drop 0.5 to 1.0 ppm in a week (if you shut off your SWCG) so lowering your SWCG way down is appropriate.

And yes, you need to add acid if your pH is truly correct.

Richard