Can't decrease space between notes while doing Cross-staff notation.

I think what you are seeing is technically the correct spacing. The noteheads are equally spaced, the left/right handedness of the stems is what make it look uneven. However, you can adjust the position of the last note in the inspector either by adjusting the leading space in the "segment" section or the x offset in the "chord" section. The leading space is to be preferred as it will also apply to all notes at the same position within the beat. If you adjust the x offset it would mean that the notehead would not align vertically with another note that should sound at the same time but as you have no such notes, it probably won't matter. For what it's worth, my personal approach would be to leave it as musescore has (correctly) written it.

Surely something would have to look strange if the notes in the example would be accompanied by 4 x 16ths with stems all in one direction in another voice or stave - either the group with ups and down stems would have noteheads with uneven looking spacing or the group with one-way stems would have actual uneven spacing.