What is a purple layer of film on the vinyl of your inground pool that you have shocked using agacide?

it is most likely an algae that has been flocked to the bottom by shocking.your chemicals have done there job now it is time to vacuum up the dead algae so it does not start to grow again much like a weed. If you keep your Alkalinity,PH and Chlorine stable you will not have to ad other chemicls with the exception of non-chlorine shock.Remember every extra chemical you add to your pool or spa is providing food for algae. kenny Kummer brody Chemical