My daughter does this sometimes too, asleep. It's silly maybe, but, I just stroke her back gently and say quietly, "No grinding teeth, Lovey. I love you so much."
That doesn't necessarily stop the grinding every time but I figure it's a sweet way to reiterate that message to her body and her subconscious.
I grind my teeth while sleeping, in times of stress. So it seems relevant to give a little physical comfort and gentle spoken reassurance/direction to someone else who is doing it! Try it and see if it helps, and if you find a solution, be sure to let us know!
Good luck, mama!