Alot of people initially treat it chemically as a algae growth major shock treatment in addition to a special algaecide.
Black algae vinyl pool liner.
This type of algae forms in a layered structure with the outermost layer protecting the lower layers.
These micro organisms develop colonies behind the liner and left unchecked eventually will permeate through to the printed side appearing as dark gray or black splotches.
Black algae in pools is similar to that which can be found in between bathroom shower tiles and on silicone seams near the bathtub.
Over the last 12 yrs i have probably looked at over 100 inground vinyl pools with this black mold growth under the liner.
Of calcium hypochlorite granular chlorine in an old sock to put on a spot.
Black algae will dot the liner in black spots.
Your marks sound like organic stains that will fade away slowly as long as you keep chlorine in the pool.
You can also use about 1 2 lb.
We think the bacteria can be washed into your pool area by flooding rains or when a septic tank overflows.
Black algae in a vinyl pool is possible but not very likely.
To remove them start with a rigorous brushing using a nylon brush.
Don t leave the chlorine on the spot for too long as it may bleach the liner.
It appears as dark black spots on the walls and floor of the pool and is slippery to the touch.
5 10 minutes max.
It s more at home in concrete plaster or gunite pools because they re porous surfaces the organism can really latch onto and even grow roots past the surface.
As long as you are losing fc overnight you still have something organic in the water that needs to be taken care of so you should continue shocking.
Bacteria sometimes show up years after a pool has been installed.
The resistant coating will become weak and break.
A few other ways to recognize black algae in your pool.
Also black algae seldom forms in pools with fiberglass or vinyl liners.
If you have black or grey cloud like stains on the floor of your in ground vinyl liner they are most likely caused by bacteria that is living in the ground under your pool.
For a semi bad part.
The problem area will lighten up for 3 5 days then come back to its original color size.
These organisms are tough and have a thick coat to resist chlorine effects.