​There is absolutely no reason for your pool to ever go green if it is looked after and maintained properly.  There are several reasons why Algae can take hold in a pool but all can be avoided with the correct water management and maintenance routines being adhered to.
Green pools are not due to the “green” coming out of the olive leaves that have fallen into the pool, or the “Olive Pollen” falling into the pool when the trees flower, they are down to poor maintenance or lack of understanding by whoever is managing the pool.
However, if a pool does go green then it is important to address the situation as quickly as possible to prevent it getting worse and therefore more time being needed to get it back to normal again. Most green pools can be brought back to perfect condition within 3 or 4 days (or quicker depending on how green it is) with the correct treatment and filtration. There really should not be a reason to change the pool water unless you need your pool crystal clear the same day so if someone is recommending a water change then get a second opinion first. With the price of water here it could save you several hundred euros.
As we have said before, prevention is better than cure. With the correct regular maintenance your pool should never go green in the first place so if this is a problem you keep having with your maintenance company then it could be time for a change.