I've been thinking this morning that succulents are green... and so is basil mixed into cream cheese.
Doesn't have to be sweet in any way. I'm not a sweetener fan, I like things with other flavors.
The normal topping for carrot cake is sweetened cream cheese. I imagine that could be used with various colourants as a buttercream alternative.
As an aside: my sister had a pork pie cake as her main wedding cake, with ornamental pigs etc. in the pastry decoration. It was fabulous!