I’ll second the dye-emitting fungus under the liner issue!
I’ve only seen it twice. Recently, a liner pool in my care was leaking quite badly and the FC would fluctuate wildly with dilution (fill water).
The leak fed a colony of fungus under the liner (apparently). Superchlorinating the water would push back the fungus via the leak and the liner would return to stain-free overnight.
The stains appear in the exact same place every time. They will not brush away. They are dark grey and splotchy (like clouds as James put it). Killing yellow algae produces a brown sediment. Killing green algae tends to produce white/grey sediment. The fungus produces an off-white, kinda pink color sediment.
We fixed the leak, mostly. The stain from the fungus was completely gone, but it returns after a heavy rain. Superchlorinating only partly reduces the staining, fades it. The streaks tend to darken and lighten with how wet the ground is and how much FC is in the water.
Try explaining dye-emitting fungus to a homeowner…
www.troublefreepool.com
