Algae can grow in colder water, albeit more slowly. If someone keeps their pool open/operational in winter, they will need to add chlorine from time to time in order to keep the chlorine in the proper range for the pool's stabilizer (CYA) level. Granted, during the shorter days and colder temperatures of winter, the chlorine demand will be reduced considerably. However, if the pool water is ignored long enough, that will eventually set the stage for an environment that is conducive for algae to gain a foothold and grow.