In the games, and also in the anime there is a hand on someone's shoulder. What is it supposed to be? The ghost of Marowak's mother?
EDIT: Not sure about that though, maybe even that is something that's not real.
Like others said, just some ghost. I'm pretty sure the dialogue in the anime is a reference to the dialogue in the game. Basically as an Easter egg. I doubt there is any extra meaning behind it, and if there is, we don't know.
The white hand on his shoulder is the hand of an unknown ghost.
The white hand is an anonymous ghost, much similar to BuriedAlive, Ghost, and the other creatures in Pokémon that are not actually Pokémon. Plus, it would not even matter if it was Marowak's mother, since the Pokedex states that Marowak outgrow their mothers.