When Stacey first arrived the other Slater girls Kat, Little Mo and Zoe were all present who all called Mo 'Nan' so it made sense for Stacey to call her Nan too as she was kind of like their sister living in their family home with their dad Charlie as her father figure. Had Stacey not been brought into the show until after the Slater sisters departures she would probably have called Mo by her name as their would be nobody around calling her Nan for Stacey to imitate.
