A Slinky (or any spring-mass system) depends on gravity for its equilibrium length, so placing it in the weaker gravity of the moon alters the gravitational force acting on its mass, changing its stretched equilibrium length. No citation is needed for this basic physical fact.
The claim is a direct deduction from basic mechanics (Hooke's law combined with gravity), making it common knowledge. None of the retrieved papers discuss the equilibrium length of a Slinky on the moon.