Hi I have a new pool (6mths). Pebblecrete finish. Was careful to ensure PH levels steady when it was really new.

The pebble is white but now has sandy stains. I can tell as there are areas of pure white and areas next to it sandy. I have tried scrubbing (no luck), increasing chlorine (no luck). Rubbing vitamin c (no luck). Cleaning doesn't remove it.

It is almost as if the pool got dirty from soil or dust and it got stuck to the pebble. It is not overly obvious but you can tell next to the white areas not impacted. When you swim down and look close, it can be seen as well and looks sandy or light light brown. It's not green or anything like that (from algae or leaves).

Any ideas would be appreciate! Thanks