I don't think it was a goof, Gregg. I think it was a loving pet name for her. And if you remember the file he kept on her medical condition was also Rosie.
I have said many times here what a beautiful episode this is. For me, it is truly magical.

I think you are probably right Cassa. Good point about the medical file. She obviously loved the character Rosie; remember when she says how Rosie performed on the night her father died, as a reason why Grace herself should continue her work. She obviously sees herself as being like Rosie and would probably like her husband to call her that.