Grass Wall

The version the world generates in surface caves cannot be knocked down and collected, so the Dryad is your only supply. That also means the two look identical but behave differently: hers, once you place it, counts as player built and blocks enemy spawns, while the natural stuff in a grotto does not. If you are sealing a cave you found, replace the background rather than assuming it is already safe.

Solutions move grass walls between the pure, corrupt, crimson and hallowed looks, which is handy when you want a background that matches the biome you are theming a build around.

Going back is lossy. Green Solution always returns plain grass wall, never the flower or jungle version you started from, so a mistaken spray costs you the original and you have to source it again. Test on a small patch before you paint a whole room.