Soap is the American name for the white dragon. It is the white dragon, not a joker and not a flower.
American cards often print the word soap. That line wants the white dragon, not any blank-looking tile.
A joker can finish identical white dragons — a pung, a kong, or a quint. A joker is not itself soap.
Chinese tables call this tile 白. Riichi, Hong Kong, and MCR all have it; they just do not say soap.
See the tiles lesson.
Soap is the white dragon, not a joker.