Mud Walls are naturally-occurring background 墙s found in the 丛林 and occasionally 地下丛林. They can be destroyed with a 锤, but cannot be picked up nor crafted. Therefore, they can never be placed or utilized in any other way by the player.
If Mud Walls are sprayed with 深蓝溶液 they will turn into "natural" 蘑菇墙s that likewise don't drop anything when destroyed. If these Mushroom Walls are sprayed with 绿溶液 they will revert back into Mud Walls or natural Jungle Walls, this seemingly being determined by 地层 level.