Many call this a theophany, which means a visible sighting of G-d, but in a flesh or visible form. What Moshe saw was different. I believe that his description of seeing his 'back' was not in the sense that Moshe saw the backside of a person, but that what he saw was like the back of a person.
Think about how you see people, from the back, they are safe to look at, you aren't being seen by them for one thing, you are seeing what they don't even see truly themselves ( not without a mirror ) and also all viable parts are located on the front.
Now 'face' them, you see now if they are male or female ( you can't always tell this from the back), you can judge their age, and the most important thing is you can see their face, which contains the eyes through which you can see into their soul.
G-d said, if you see my face, you will die. ( Exodus 33) That is because therein lies all the power, just like in a person in his face, in his eyes lies his.
As a child we do things behind our mothers or fathers, back, no?
Moshe was hidden from seeing all of him, he was only allowed to look once he had 'passed by' and only saw his back, saw him leaving, not coming towards him.
What Abraham saw was G-d in a human form, one that could walk, and talk and eat with him in the plain of Mamre.
The other times would be when Jacob wrestled with a 'man', and when Joshua came upon the man with the sword who told him he was the commander of G-ds army. Joshua worshiped him and was not told not to as others that are angels would have. ( Joshua 5:13)
Another time would be in Persia when in the court of Nebuchadnezzar the three scribes with Daniel were thrown into the fiery furnace for not bowing down to the golden image , the fourth one was proclaimed by Nebuchadnezzar himself to look like the son of G-d.