Rust - Why and how to repair

I believe what you are seeing is condensation trapped between the wall and the liner. My pool had the same issue before it failed (see link in my signature). How old is your liner? In order to fix this you will need to drain the pool and remove the rust. If the rust is all the way through on the bottom of the wall then you will need to replace the wall since the bottom edge cannot be repaired. For rust on the side of the wall you can sand that off. Very small holes can be patched with an auto body repair kit such as Bondo. Anything larger than a fist will need to be patched with sheet metal riveted to the pool wall. There is some more information on repairing pool walls in this article. Advanced Wall Repair for an Above Ground Pool

I can guarantee that any rust you see on the outside of the wall is far more extensive on the inside of the wall. Pick a time to do the repair when you can complete the job in 2 or 3 consecutive days. You will need to take the pool completely down to repair the rust. Take photos of everything while you are in the process of taking down the pool so you can remember how to put it all back together. Cover the pool site with a tarp to protect it while the pool is down. The liner will probably need to be replaced, but if it is fairly new you may be able to store it in a Rubbermaid container with some water to keep it from shrinking. You will probably need to patch the skimmer and return openings and cut new ones, as it is very difficult to get the holes to line up again.