I was vacuuming the pool this evening prior to the pool closing on Monday. There were a few leaves in the shallow & deep end for a few days, but nothing major. As I am vacuuming the deep end grabbing all the leaves I see something small which is not being sucked in through the vacuum, so I shut the filter off and looked closely.

It is a small spot, maybe the size of a quarter that looks like rust, something brown (hard to tell). I grabbed my brush and brushed the area with no success. I used a flashlight and the area looks like an area in your home where the sheet rock and nail or screw start shifting, like an indent, but it is very hard to tell as I am not looking at this head on. The area is a on the wall, about 2 inches before the wall slopes down to the main drains about 2.5' below pool water (NOTHING CAN STAY ON LINER) so what ever I use has to be scrubbed.

There are no other stains on the pool liner and 2 minor dots on the steps. Originally I had the Frog system, but this only lasted about 4 months this year as my pool was opened up real early (long story). The Frog has been removed for a while and this stain was not here a few weeks ago. I am pretty good as spotting any problems.

What could this possibly be and can I use vitamin C to remove this. I can not get my hand down there even if I lay down on the concrete. I can probably get down there with a small soft too brush. The liner is dark and I do not want to do any damage to the liner. BTW, I have tested for metals every few weeks and there is nothing lurking in the water based on the pool company results. I do my own testing and my levels have been perfect from CH, TA, CYA, PH and FC, CC and TC.

What can I purchase and attempt to remove this before the pool is closed on Monday. Thank you.