Rust is really iron in the water. To remove rust or iron in a fiberglass swimming pool, you just need to add a CuLator Ultra 4.0 into your pump basket and let it work all summer.
If you are using well water to top off your pool, then add a maintenance dose of Metal Sequester to the water when you are topping off.
If you have rust, iron, or other metal staining, use the No-Drain Metal Stain Eliminator Kit to remove the stains without having to drain the pool and do an acid wash. Acid washes are very hard on the surface of the pool. Try a No-Drain Metal Stain Eliminator Kit first to see if that will solve your problem.