You may not be able to remove it with stripper, but if you are very careful to lay the edge only on the flat, you could try using a glass scraper. If you do end up with some scratches in the glass, they won't be noticed after you install another background.

I've bought several breeder tanks that had been painted and have never had any problem scraping it off with a razor blade.

However, if that first layer was fused to the glass, you may need to cover it from the inside, and the 100% acrylic paint used to paint resin aquarium decorations is just as safe to use to paint the interior of a tank. You just need to let it cure completely; meaning at least 72 hours, then fill the tank, let it sit for a day, then drain before refilling it for the fish.

I know a fellow who wanted a 3-D background, so made his own, using acrylic paint, slate and rootwood and fixed it all in place with aquarium cement. He even made pockets for plants on the back wall. The end result was spectacular.