This is one of those weird rumors that refuses to die. It's become part of pop culture. In fact, when you search Google for "Great Wall of China seen from the moon," you get over 4 million results. The most interesting part is that it's a fake fact that got started way before humanity dreamt about going to the moon.
A man named William Stukeley has the first known appearance of this myth. One of his letters written in 1754 says that " the Chinese Wall, […] makes a considerable figure upon the terrestrial globe, and may be discerned at the moon."
This fake fact even appeared in a Ripley's Believe it or Not strop, and the Second Book of marvels. In fact, in order for the Great Wall to be seen from the mone, it would have to be over 70 miles in diameter, and have to have significant contrast with the areas surrounding it. The Great Wall is at most 30 feet wide, and of similar color to the soil around it.
To put it in context, the width of the Great Wall as seen from the moon would appear the same as if you viewed a human hair from 2 miles away. That means you would have to be able to see 17,000 times better than a 20/20 vision!