“A subsatellite, also known as a submoon, moonlet or informally a moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet”
It might be helpful to describe gravitational relationships with human generational terms of parent-child, grandparent-grandchild, sibling, cousin, etc. So the Sun is the Moon's gravitational grandparent and Jupyter is its parent-sibling (Aunt/Uncle, maybe pibling upwards and nibling downwards).