My parents tried to name my son something I had not named him. I gave him a name which included a first name, middle name and last name. The middle name is my father's name, and as far as I'm concerned, his Indian name. (I am Indian, although my husband is not.)
For some strange reason, my parents didn't like this, and gave him another Indian name. I was okay with them using it for formal situations, like when we had a traditional party for my son when he was around 9 months old. But initially, they just used it ALL THE TIME. Whenever we talked on the phone (we live far away from each other), they would ask after my son using the name they gave him. I let it go for a few months, and then asked my mom why they didn't call my son by his actual name. My mom asked what I meant. I said "you're not using his real name...you're using the name YOU gave him which is not his name". My mom hung up on me real quick, but they stopped using that name.
It was easy for me because we don't actually see my parents that often, only 2 or 3 times a year. Otherwise, we skype or talk on the phone. If you'll actually be spending a lot of time with your mom, I would simply correct her. It could be awkward, but honestly...grandparents don't have any right to name a child, unless they were specifically asked.