If it is gunite definitely don't shock it! Someone I know did and it left white spots on the grey plaster bottom.

Cold temps only reduce the chlorine demand. Below 55 degrees, salt cells stop producing. Many people use Clorox to augment the free chlorine level. Filter cycles may be shortened except in freeze protect modes.

Please remember, algae is only one of the many badies that can invade a pool. Keeping the FC, pH, Alk, and CH are still important.

Using powdered chlorine may add undesirables such as extra CYA or calcium and tend to be slow to dissolve. Using bleach adds only a little salt and has no solubility issues.

Scott