Do you believe that the story with the burning bush is true? if that is the case, didn't God seem to take the shape of the bush or something that could be described as being physically visible to the naked eye?
Exodus 3:2-6
2 - And the angel of the LORD appeared to him in a flame of fire out of the midst of a bush. He looked, and behold, the bush was burning, yet it was not consumed.
3 - And Moses said, “I will turn aside to see this great sight, why the bush is not burned.”
4 - When the LORD saw that he turned aside to see, God called to him out of the bush, “Moses, Moses!” And he said, “Here I am.”
5 - Then he said, “Do not come near; take your sandals off your feet, for the place on which you are standing is holy ground.”
6 - And he said, “I am the God of your father, the God of Abraham, the God of Isaac, and the God of Jacob.” And Moses hid his face, for he was afraid to look at God.
Otherwise it wouldn't make much sense for Moses to be afraid of looking at God or the bush or whatever form God decided to represent himself as in front of Moses.