Towards the end of Summer for e past few years I start getting a reen slime at the bottom of my pool what could be the cause of this problem . I constantly vacuuming it
This could be due to a number of things, but is likely due to an insufficient chlorine level. The lack of chlorine could be due to a build up of demand over the course of the summer or quicker depletion in high UV and/or high bather load situations. Monitor the free chlorine (not total chlorine) and pH levels at the end of this season and you should see improvment. You may also consider a weekly dose of algaecide (called a maintenance dose) to help support your chlorine demand.