Clean your pool once a week. Cleaning includes vacuuming the bottom, skimming the top, emptying out the pump and skimmer baskets, balancing out the chemicals that make your pool water clear and brushing the walls of the pool.
The best way to deal with discolored plaster is an acid treatment. It is less expensive than re-plastering the entire pool. Wash all of the plaster with a mixture of water and muriatic acid. This process eliminates the tough stains and yellowing of the pool.
This job requires opening the filter and rinsing down all the grids and the inside of the filter. Then check every grid carefully to make sure there are no cracks in them and no debris is being returned to your pool.
It can take between two days and two weeks to remove the algae, depending on the type and extent of the algae infestation. Shock your pool with high levels of chlorine and algaecides to kill the algae in your pool, and vacuum the dead algae to leave your pool clean and safe.