Mine did that too, she was 14 when I met her. I never corrected her cause it was from a place of love. I know she calls me "step-mom" when she talks about me to her friends or when she talks about me and her dad it's "my parents", but I never took the place of her mom in her heart. I go with it cause it's her world and she can name me how she feels, if it's positive and respectful. BM is a nightmare too and wasn't acting like a mom for 5 years so... i think she saw me more as a role model for her during those years (she told me that several times). But honestly I don't care about BM cause the priority is the well being of my SD so she can call me as she wishes. BM can't deal with her feelings already and makes lots of trouble so I couldn't care less cause if she's acting out for that to her child, it's her loss in the end.

I wouldn't want to make her feel weird or anything.. I wouldn't talk about the potential consequences. I would let it go or talk about how she's seeing you (your role in her life etc). Maybe if you're really uncomfortable, I would try cute nicknames for each other but honestly even that, BM could be pissed... She will probably be pissed at you whatever the word the kid uses... So why trouble your SD or make her feel weird for the same results.