“Nerd juice” is meant as a non-literal insult. The idea is that a nerd/geek, like a snail, leaves a trail of slime/juice behind him. So, it’s a way of insulting the nerd by comparing him to a low form of life. But the nerd “got even” by tricking the bully into thinking that he had something on his face, making the bully look and act dumb.

Thank you, James! You should've posted your explanation in the answer section. It's perfect.

Hi Nikita, 'Nerd juice' doesn't mean anything at all; the bully is just looking for a fight and is trying to insult Ed by calling him a nerd, but the 'nerd juice' is not real and is purely an insult. I suppose the idea behind Mark's insult is that Ed is such a nerd he leaves a trace of his neediness wherever he goes; he contaminates Mark's territory with his nerdiness. The idea of a human secreting juice is also quite grotesque, which adds to the insult. In short, this is childish nonsense from the mouth of a bully. Hope this helps!