Verte chaude would be grammatically correct, though if there's just a dash of Chartreuse in a cup of hot chocolate, I'd wager the gender of the most statically significant beverage should prevail (Vert chaud).

Also chances are you'd drink those from a mug or cup, not a glass.

Well that's what happens when you disturb the natural order of things by mixing hot chocolate and Chartreuse ;)