What did Moses encouter on the mountain?

"No one has seen God at any time; God the only-begotten Son, who is at the Father's side, He has made Him known." - John 1:18

No one can behold God in His glory, as God Himself says, "No one can see Me and live" (Exodus 33:20), which is why Moses was only permitted to see God's "hind parts", exactly what that means is probably too much for us, but it does seem to suggest that Moses was permitted to experience a small measure of the Divine glory, sufficient in itself to cause his very face to shine like the sun and require him to wear a veil across his face.

That's why we don't meet God in His naked glory. Consider those who tried to touch the Ark of the Covenant when it was being brought back from the Philistines, they dropped down dead from the simple touch of the Ark.

The Scriptures are replete with this fact: The Naked God, that is God in His Glory, is exceedingly dangerous and terrifying, and men could only behold God behind veils, and even then He was terrible and terrifying--as fire and smoke and thunder.

No one has ever seen God, no one has ever beheld God purely as He is. Men have only ever experienced the terrifying glimpses of His Incomprehensible and Terrible Majesty. "Terrible", not because God is vicious, but because mortal man cannot comprehend, fathom, or deal with the Living God because He is so far beyond and above all things. This is why God has always used mediation, and that is why we have the clearest picture of God, not through the veils of fire and smoke, but in the flesh of Jesus Christ. So that to see Christ is to see the Father, "If you have seen Me you have seen the Father", for God has willed that He be known through His Son who became flesh for our sake.

For this reason the Apostles beheld God more clearly than Moses, for Moses may have beheld the merest glimmer of God, but Moses could not know God the way the Apostles did. For the Apostles met God face-to-face, they beheld God who met with them, ate with them, spoke with them, lived among them, heard God speak with their own ears, saw God's miracles through His own hands, and they saw God crucified.

The Prophets knew of God, but even Abraham looked forward to Christ's day, for the Word became flesh, God's feet walked upon the ground, His feet were upon the Mount of Olives and He spoke to the masses as a man.

-CryptoLutheran

God can have many ways of communicating with humans. Some we can comprehend, some not.
To Moses it's face to face communication. However the point is, He doesn't need to show up to communicate. He can remain hidden and transparent when He communicates with some one on a face-to-face bases. He doesn't need to show any image to achieve a direct communication.

That is to say, you don't need to apply your sense of what a face to face communication is to how God communicates with Moses. In our concept, the two side must show up with an image in order to achieve a face to face talk. God however has all the ability to do someone beyond our conception and comprehension. The simplest I can tell is that all God needs to do is to first confirm that He's God but no one else, then He can speak with Moses using audible voice but without showing up.