These are extreme cases of not cleaning your pool. In order to make them usable again, they'd have to be drained, scrubbed, sprayed down and refilled. After that the pool professional would have to put in the necessary chemicals to balance the water chemistry. The easiest way to avoid your pools looking like this is to hire a professional pool cleaner! These are neglected pools.
The third photo is more common. It turned green due to little or no maintenance. It would require a chlorine shock. The pool would also have to be checked for any phosphates or metals in the water, anything that algae would grow on.
When there isn't enough chlorine in the pool, algae will start to grow. Especially in areas where the sun doesn't reach. If the PH level isn't stabilized the chlorine will not be able to kill any algae. This will cause the water to turn green or yellow.
Another reason for an algae bloom would be an object falling into the pool. Trees in the surrounding area will dirty up the pool, some types of leaves and bark will cause algae to grow and can even create "organic stains" on the plaster. When stains are present more shock is needed on the stains as quickly as possible, and a good scrubbing of the stained area to get it out. It is also necessary to remove debris, leaves, etc if noticed in between pool cleaning days.
The two most important aspects of keeping a clean and safe pool are the water chemistry and the filter. Filters need to be running no less than six to eight hours during the day to maintain proper water turnover rates, ensuring the water stays clear and blue.
