It's pokémon, so it's probably Shuckle* juice or something.
*Which has never been outright confirmed to be alcoholic, kid friendly franchise and all, but it's heavily implied since it's the fermentation pokémon.
I really like the detail that Marcille is the one backing away in panic. Falin isn't grossed out, just surprised (because she can never figure out people are in love with her, this has been established).
This ship has definitely gotten more popular after the anime, and it's a cute one. Mostly comes from that one bonus page where Kiki triggers Namari's bisexual panic/fetish for human leg pouches.