Hi Oasthouses,
The stain maybe unsightly but that does not mean a new liner. I am not surpised that algaecide did not shift it as the algaecide most commonly sold in France is pretty weak stuff.
Chlorine is far better at removing stains. After a storm? could it be dirty water being washed under the liner and appearing as a stain? Or have the farmers been cutting the fields recently?
It is going to be difficult untill you actually get there and can take a look, what have you got under the liner is is the usual felt backing, sometimes that gets growth on it but usually after the liner has been removed for some other reason and light and air have got in.
What region of France are you in?
What type of pool is it, salt water or direct chlorine, what type of filter do you have?
The tests you need to carry out are:
PH, Free chlorine, Combined chorine, Cyanuric acid, Total alkalinity, Calcium hardness and a good idea to check the phosphate level.
The volume of water in your pool?
Yes I would prepare to shock the pool so pick up some 9.6% Javel from a Brico 15 euros (ish) for 20 litres.