It's supposed to be his true form, that's when he was incubating and trying to prevent surfer from prodding into his mind. Recall when Norrin told him to stop hiding behind manifestations of animals and to reveal his true face. It is possible to argue that Isn't his true form as Galactus, since Lifebringer one hadn't been "born" yet and could just be him taking on the form of what he knows at that time. That being his abstract existence as pure destruction and consumption.
Though, his actual true form as of his incubating period is what he was before Galactus, Galan of Taa.