There are some cases where adding polyquat during an algae bloom has helped, but the general consensus has been that it's much better as a preventative than to kill an existing bloom. I understand that it tends to drastically lower your Cl after the dose, and that your filter needs to be on recirculate instead of filter for awhile, since it will eventually filter out, so it would make sense to me to add it after your bloom is cleared up.

However, this is only my opinion--my pool stays open year round, so I don't use either polyquat OR a cover, so take it with a grain of salt!!

Janet