I used the StainFree/MetalFree combo at several of my pools when I took over their operations. Some had green staining that had been building up for years.
The worst one, at 20,000 gallons (75,000 L) took five bottles of StainFree and a lot of elbow grease to completely remove the stains.
Depending on your pool's finish, consider using one of the pool brushes with the wire bristles rather than the softer ones. They can damage the finish (they tend to take off the paint in our concrete pools, and scratch the glazing on the tiles, if they are used too much), but in heavily stained areas they can help to lift it off more quickly.
Interestingly, I only have these green stains around the outside of the pools (floors only, not on walls), typically in shaded areas only, and they were only on indoor pools using SWGs. I wonder if the salt has something to do with it?