From the Pilot episode of Rick and Morty, we've seen Rick Sanchez almost constantly wearing what looks like large drool stains on his lower lip while spitting sputtering, and burping in every other sentence. But what exactly is that green stuff in Rick's mouth?
The Green Stuff on Rick's Mouth is Drool
When you look a little closer the liquid isn't necessarily that green. It's no secret that Rick is an alcoholic, the very first scene in the Pilot involves a drunk Rick stumbling into Morty's room and dragging him out to his spacecraft. Once the spacecraft lands again, we see a huge amount of different beer bottles stumble out with him making us believe he has continued drinking on the flight.
There are numerous other references to his drinking throughout all the seasons of Rick and Morty. The Pickle Rick episode is an entire metaphor for Rick's alcoholism and literally being "pickled". There are more direct references such as in the episode of "Ricksy Business" when Rick tells Summer that his only reason to party is "TO GET RICK-ETY RICK-ETY WRECKED SON!"
It is most popularly believed that the green stuff in Rick's mouth is nothing more than drool from his mouth. That his overconsumption of alcohol is linked to his inability to control certain bodily functions and lack of self-awareness so that he never cleans himself up. However, there is an alternative theory as to what the green stuff on his mouth is.
The Green Stuff From Rick's Mouth Is From Mega Seeds
In the Pilot episode of Rick and Morty, Rick gets Morty to sneak mega seeds from dimension C-35 in his anal cavity. As a result, this gives Morty superior heightened intelligence where he can calculate the square root of Pi and quote the first law of thermodynamics.
However, this level of genius is short-lived and Morty begins to lose motor functions in his body. But the thing that fans notice the most is that Morty begins to "drool" exactly like Rick does the majority of the time.
It is hypothesized by fans that Rick is the smartest man in the multiverse because he is constantly using the mega seeds to enhance his intelligence. On top of this, the contents of his hip flask isn't alcohol but actually mega seed juice in an attempt to stave away the withdrawal symptoms. Therefore whenever we see the drool on his lips, it's actually because he is about to start withdrawing from the mega seed juice which is why we only sometimes see him with the green stuff on his mouth.
Although this is a spectacular theory, this has not been confirmed by the Rick and Morty creators.
Related: Rick and Morty Season 5: Where to Watch and Stream the Entire Series (July 2021 Update)