Hi! and Welcome! You've gotten excellent advice (as usual from Jan and Lisa). I don't have much to add except:

Personally, I'm biased against flocs and clarifiers. The only one of the latter I like is also the only algaecide we recommend, Polyquat 60%. But I would NOT recommend it if you are going to drain and refill until after you've drained, or you'll just be pouring...down the drain.

But, unless you are vacuuming to waste, after you refill it's fine to add Polyquat 60%. It will tend to push your chlorine level down for 24-48 hours but it will inhibit new algae growth and acts as a clarifier as well.