In response, Charlie said, "You're welcome, Grasshopper" as he slammed the door in Alan's face.
For us in the Trueniverse audience, the mention of "Grasshopper" was supposed to be a call-back to the TV series 'Kung Fu'. When he was a boy, the main character Kwai Chang Caine was often called "Grasshopper" by his teachers at the Shaolin temple. Its most famous, and most parodied, use was in the phrase "Snatch the pebbles from my hand, Grasshopper."
(I said "snatch". Heh heh. Heh heh. Heh heh.)
But for the inner "reality" of Toobworld, there should be a different reason why Charlie called Alan a grasshopper.
And I think it's just because Alan reminded him of the insect......