Do you mean this one:
I see the moon and the moon sees me, the moon sees somebody I want to see; God bless the moon and God bless me and God bless somebody I want to see.
I think it's almost a common memory.
The kind of thing everybody knows and recognize,
but nobody knows who wrote this or where it comes from.
in the movie it's supposed to reflect Ann's state of mind:
On one hand full of regrets, on the other feeling that she is less alone than she formerly felt.