So, she banged him, had a child, and now that child's child is visiting.
I think with the stole the guy found, it's a bit of a bent on the Selkie fairy tales of the fisherman hiding his wife's selkie skin to prevent her from becoming a selkie again.
I'm gonna be honest. I found this to be unnecessarily depressing. Like the mangaka deliberately found a way to make this story sad. It really shouldn't have been.
Why do I think this? She was obviously carrying their child. Why leave? The call of nature and her flock too much? At least leave the kid with the dad and visit once a year for goodness sake.