She doesn’t spot him, so wrapped up in the song as she is, until well into the music. She startles when she catches him; the music hesitates for half a second before picking up again, and she’s singing directly at him for a moment, and then to him. Rose finds it nice.

She can’t really say anything or make any gestures, so she smiles to try and say he’s welcome to come in. Whether the message actually got conveyed or not, at least he'll know she doesn't mind him.