Recurring Stain

I have a vinyl 25,000 gallon oval inground pool in Pensacola, FL. In the deep end side wall, I have an irregular shaped 1' X 3' grayish-colored stained area. The spot will not scrub off but disappears after I shock with liquid chlorine.... for a week or so and it returns again. For a while, I considered it a useful "early warning system" until a second spot appeared several months later about the same size. Any ideas on what the stains are and how to get rid of them? Thanks

You have to post your numbers, free chlorine, total chlorine, ph,calcium, alkalinity,and cyanuric acid. This way we can tell better what is going on. The fact that the chlorine gets rid of the stains makes me suspect that your chlorine level is not high enough for your cya, which will make algae grow - especiall that it is showing up in spots that are not getting the best circulation.