Vinyl liner feels like and looks like sandpaper

20x40 inground diving pool. Liner is 3 years old.

I've been a pool owner for 14 years and this is the first time I have experienced this. Took the cover off the other day and put everything together to start vacuuming out the dirt and as i was cleaning i noticed a 8-12 inch portion at the bottom of the walls that felt rough.

The top portion of the wall is normal and the bottom of the liner feels normal. It goes all the way around the pool and feels like and looks like sand has been glued to my liner. Had to remove the light below the diving board because the bulb burned out (it was the original bulb) and when I pulled the light out the bottom portion of the light had this sand like material on it also. I was able to scrape it off but it took some effort. One again it looks just like SAND. Brushing the wall with the brush doesn't do anything and the water is to cold to keep my arm stretched out scrubbing with a scratch pad. Been doing this for 14 years and yes all chemicals are in range. I take a lot of pride in my pool but this has me baffled. Thinking I will have to wait til the water warms up and just take my time rubbing it off. Any ideas anyone?

thanks Brian