This is one of the victims of the Vashta Nerada, an alien carnivore shadow-like life form which invaded The Library from the Doctor Who episodes "Silence in the Library" and "Forest of the Dead". The Vashta Nerada attack their victims by pretending to be the shadow of the victim (many times this leading to the victim to have two shadows, one being the real shadow and the other the Vashta Nerada), and then "eating" the flesh. After that, they can even control the remains of the victim (in this case, the skeleton insisde the space suit).