In Season 1, I want to say around episode 4, where Tilly and Burnham finish a run through the ship, Tilly wants to order a green smoothie with "extra green". Burnham cancels the order, asking for something healthier instead. What is the "green", and why would one want extra of it?

In Discovery episode 6 ("Lethe") Tilly and Burnham finish a run through the ship, Tilly wants to order a green juice with "extra green". Burnham cancels the order, asking for something healthier instead.

Tilly: Computer, green juice, extra green.

Burnham: Computer, cancel that order. That's hardly the correct ratio of protein, carbohydrates, and fat. Computer, three egg-white breakfast burrito with black beans."

What is the "green", and why would one want extra of it?