If your swimming pool is green you're going to want to shock it firs,t to kill the algae in the pool. What usually happens next is that it turns white, and is very hard to filter out.
I used flock made by Blue Shield to help clear our swimming pool up. The flock attaches itself to the algae and impurities in the pool causing them to sink to the bottom so that it can be vacuumed out.
If you have a sand filter you can add it to the pool or the filter itself, follow the directions on the bottle. This stuff works very well.