It sounds like you're not completely killing the algae off when you shock the pool, and then when your chlorine drifts back down it just comes back again. When you shock your pool, you need to hold it at shock level until you can go overnight without any chlorine loss, and don't have any more green left in the pool. This sometimes takes a few days, so if you're just shocking for one night you probably aren't killing it all off.

Janet