It's a mixture of what she wants to show to people and what people want to see. She was killed after being nearly raped by Constance's husband; she might think that all men are "the same" and thus she shows herself as a super-sexy-vixen to mess with them. Young Moira is very sexual, also she doesn't have her gunshot wound. After a while Ben starts seeing her as an old woman and she days "Congratulations Dr. Harmon, you're finally seeing things for what they are".
On the other hand she is nice and compassionate with women of the house; she shows herself to them as an old and kind maid, so they feel reassured. I can assure you if Vivien had seen young Moira, she wouldn't have hired her XD also old Moira has the injured/blind eye from Constance shooting her.
Ghosts have at least SOME influence on how they're seen by the living, Norah doesn't always have the wound to her head, for instance, and Tate doesn't have the scars/wounds on his chest, otherwise Violet would've noticed them when they had sex together (also afterward we saw them in her bed together and his chest was intact), the Black Dahlia doesn't always have the knife-made smile, etc. And since they can be invisible too it's safe to assume they can toy with their appearance.
Thank you so much for clearing that up. I suspected it was something like that and you're absolutely right no married woman, having trouble in her marriage or not, would hire a maid who looked like the young Moira. :-)
What do you think?