People are concerned about his because that is what we see when we take a bubble bath :-) Stains are created when the soap bubbles are used to wash our oily body and as a result have a layer of dirty foam floating.
In the case of filling soap liquid foam in PyraPOD, the double layer cavity is sealed and no dirty or anything can get in. It is clean there and also clean in soap liquid tank. Also, the cavity is wet whenever there are wet foram and liquid film in the cavity. Even there are stains on the inner skin, they will be washed down with the bubble or liquid flow. No chance to form dry stains.