Do not drain it
It is the logical reflex and the worst decision. An empty pool loses the water's counterweight against the ground, and in autumn-damp soil the shell can lift. The lining also dries, shrinks and cracks.
Green water is recoverable. A lifted shell or a cracked liner is not. Before draining for any reason you need to know the water table and the season, and November is precisely the wrong one.
What gets done, in order
Remove the leaves from the bottom and the surface, because they are what keeps feeding the problem. Clear the skimmer basket and the pump pre-filter, which by now are usually full.
Then a shock treatment and continuous filtration for twenty-four to forty-eight hours. And backwash the filter when the pressure rises, which it will.
Why it turned green as the heat dropped
Because the pump stopped running, or ran far too little. Algae do not need heat, they need still water and no disinfectant.
In October, on two hours of filtration a day and with nobody adjusting the chlorine, a pool that was perfect in August turns green in ten days. It is not a fault, it is a missing cycle.
Active or passive winterising
It does not freeze around here, so active winterising is the norm: filtration kept running a few hours a day, pH adjusted and the vacuum passed every fortnight. That is what lets you open in April without a shock treatment.
Passive winterising, with the plant shut down and a cover on, makes sense if the property really is closed until May. But it needs preparing properly: treated water, level dropped below the skimmers, and a cover that will take the wind.
What leaving it until March costs
Green water treated in November is a few days of filtration and product. The same water abandoned until March brings deposits on the floor, stains embedded in the lining and sometimes a pump seized from not turning for five months.
The gap between those two bills is what separates maintenance from repair.
