After the four large Galilean Moons of Jupiter the next brightest moon is Amalthea size 270x170x150kms and 14.1mag is beyond the range of most backyard scopes as it is so close to the planet as to be overwhelmed by it,
the next brightest is 16thmag Thebe, so unless you are using a very big scope or maybe CCD imaging what you observed was not a moon of Jupiter.astroron
