Crimson Walls are naturally-generating background walls found in the Crimson. These naturally occurring walls can be destroyed with a hammer, but will drop nothing. These walls also do not stop enemies from spawning and cannot be used for proper housing.
Some cave walls can be converted into crimson walls: Mottled Stone Walls become Crimson Teeth Walls, Fractured Stone Walls becomes Crimson Blister Walls, Worn Stone Walls become Crimson Crust Walls, and Stalactite Stone Walls become Crimson Scab Walls. Using a Clentaminator with Green Solution is the only way to reverse the conversion.