it sounds like you didn't finish shocking- when the green color goes away, it means you're beating the algae but there is still some lingering. The overnight drop test (lose <1ppm FC overnight) is how you know the algae is dead- you need to maintain shock levels until you pass the overnight test, and have <.5 CC (TC - FC)...
when you have algae, it consumes your FC faster and then the algae takes over- it can take a lot of chlorine to kill a batch, if at all, if you don't maintain the shock levels...