Ah, this one is hard for me to answer because I called both of my grandparents "grandma" and "grandpa" on both sides of the family.
I think it is a good idea to just reassure your mother that your daughter is at an age where differentiating can be difficult - I've known plenty of kids for instance that have called every single animal on the planet a "dog" for a while, because they're most common with their family dog. It could be seen as a compliment that she's calling her other grandmother mom mom too - maybe try to emphasize that point of it?
