Black ring around top of pool 3” above water line

[Us-stralya]

Hi all
Thanks for letting me join the group, I have never owned a pool before and we bought the house and the pool has got a blackish dark ring around the top of the pool about 3” above the water line, it’s not a mould as mould remover does nothing, can anyone help please seems to have a calcium build up too
Photos are hard to see it but you can see it

 

[ajw22]

I suspect that at some time the water level was kept at the level of the black ring.

The black ring looks like a waterline scum and scale line. Usually ceramic or glass tile is placed along the waterline but your pool looks like you have pebble plaster up to the coping. Tile is used at the waterline since it is easier to clean and you can be more aggressive cleaning tile without damaging it. I think you will damage the plaster with anything that will clean the dark ring.