I have an inground pool with a vinly liner I have a black stain on the side?

Answer 1

Take a sponge with one of those 3M scruby pads on
one side, or wrap a 3M scruby pad around a sponge
and dive into the pool. Go under water to the site of the
black stain and give it several good scrubs. If the stain
begins to come off, you know what you have to do, and
that the stain is on the same side as your scrubbing effort.

If the stain fails to be changed by your scrubbing action, it is very possible that the problem is on the other side of your vinyl pool liner. If so, there are vinyl paints that you could look for and use to cover the stain sometime when your pool is empty for cleaning. Be certain that the paint is compatible with vinyl plastic.

Answer 2

I hate it when this happens it sounds like you have black algae. I would say you need to "shock" your with chlorine. Your local pool place can tell you how much you need to add just take them a water sample and tell them the size of your pool.