And this is interesting question about Moorwraith, there's no word "terrain" in his aura. I think that Grave Goo can affect only terrain pieces\markers.
One more question: if Grave Goo in Web Marker and in the Forest then both terrains gains Hazardous trait. And both of terrains are separate, so what about enemy models like in this situation? Will Hoffman suffer double Hazardous effect?