"Mermaids" in Japan fall under the yokai classification. Yokai can be translated as "bewitching spectres". Although some yokai have human shape, the animal part of the creature almost always overwhelms the human. Two that stand out:
Kappa: humanoid demons or imps that lurk near water.The lack of mermaids with beautiful upper human halves in Japanese lore may be explained by two factors--the existence of real, female pearl divers in Japan (why go supernatural when you have the real thing?) and the fact that so many Japanese gods/dragons (who occasionally take human form) live near or on water to begin with. (Water is rather difficult to ignore in Japan.)
Ningyo: part human/part animal--not the sirens of Western literature yet still supernatural since the flesh provides long life.
Pearl diver in Japan
The connection between amorality and merpeople is fairly old and seems to be connected, at some level, to the trope of the sea as a changeable, indifferent force that cannot be controlled or understood and doesn't play favorites.