Probably butte cream. However I recently did a version of this cake, and I extruded yellowish fondant for mine. Lined it all up haphazardly against a BC iced cake, and then brushed it with some darker yellow and brown food color powder for shadows and dimension.
Ummmmm, that should say butter cream.
That seems to be hand piped butter cream to me. I can't think of a proper name for that tip but there's one that creates like 6 #2 strings. It looks like they used that and then came back with a single tip to add the randomness.
It looks like they used BC and a grass tip and went up the sides of the cake and on the top.
I think the large tip #233?