Can anyone help me with what this might be on our above ground pool liner? We have hard water from a well and cant seem to get this "stain off" for the past 2 weeks after opening and pulling the cover. the local pool store has told me to shock and brush but this is not doing anything but eating up my money and not the stain lol. The water was tested and Alkalinity is high as well as Calcium (cant change that do to well water) .My husband got in our cold pool and attempted to scrub this off with a towel and foot power and it didnt even budge. I have tried Vitamin C and it does nothing. If i brush it in some spots its like a plume appears and in some spots it looks like sand piles even after vacuuming it. The surface is rough where the color is