I noticed that too, but going back a few frames, you can see that the shape is also part of the falling water, so we may be mistaken there (Or it is the antennae and being part of the water is an artifact from editing Mothra's body out of the scene). Either way, I'm sure they'll give her antennae. If the head is the adult form, they might just be out of the frame or they might temporarily be flat against her head. EDIT: I JUST thought of a way that could be the larva. Millie Bobby Brown's character is supposed to have some psychic connection to the monsters, right? Maybe Mothra isn't really there in that scene and it's just a vision that only she can see. It might be a stretch, especially given the broken bridge around them, but it's a possibility.