How do you clean green algae of the bottom of a pool?

There are a wide variety of algaecides (algae killers) available. Your local home improvement store, or pool store, has them. Most pools can be treated for about $20.

You will want to use an algaecide, and "shock" the pool which briefly brings the chlorine level up to an insane amount, and then use a clarifier which will cause all the dead algae to clump up and sink to the bottom, where it can easily be vacuumed out.

All of these chemicals evaporate in a day or two, leaving the pool ready for swimming.