In the Shia Islamic belief, Allah is not confined within time or space. To confine Allah in a form or within a time frame, would limit him. This is no different to the Christian beliefs which state that God came down in the form of a person (Audhobillah). This belief is also similar to the Salafi beliefs where they compare Allah with his creation and give him physical movements and parts.
Therefore, we wouldn't say that Allah literally came down to earth rather any communication to Prophet Musa (peace be upon him) would of been through a type of creation from Allah. We can say that this incident was a manifestation of Allah's power and glory.
Furthermore, when Allah told Prophet Musa (peace be upon him): "You will never see Me" 7:143 this applies in both this world and the hereafter. This is unlike some other sects who believe that Allah will be seen on he day of judgment; for the Shia belief, Allah cannot be literally seen otherwise this would necessitate Allah being in an image and form.
May Allah grant you success