I assume so. It is likely that Polteageist's body is actually gas/liquid taking up the shape of the teapot, Like real liquids. On the other hand, the head, arms, and legs are the weird parts since they're leaking/stretching out of the body.
Why/How does it's body stay and take the form of the teapot? Well its a ghost Pokemon. Other than that, no one knows for sure, but when the game comes out we'll be able to check its Pokedex Entry and if it has one, relate it with the design of its evolution. However yes I do believe the entire material inside the teapot is part of Polteageist, which explains its fluid and liquid like animation.