Darrell, of course you can shock the heck out of it. The bleach will be a little more effective at the lower temp If the filter system has been winterized, you'll want to keep brushing the floor/ walls and the algae will just die and lay there. If the system is still up and running, shock, brush and either vac to waste or just vac. The colder water is going to aid in the process, the algae is slowed down and the chlorine is more efficient. Just be sure to shock to the recommended level and keep it there until the algae is dead.