海卫 is a naming prefix, not used as a standalone word. Chinese names moons by their host planet's first character +
卫 (wèi, 'guard/satellite') + a number:
月卫 doesn't exist (the Moon is just
月), but
火卫一/
二 = Phobos/Deimos,
木卫一/
二/
三/
四 = Galilean moons,
海卫一 = Triton.
