A possible clue - on the Calibre metadata page, there is a field for 'author sort', and if you hover the mouse just right, a popup appears that says:
"Specify how the author(s) of this book should be sorted. For example Charles Dickens should be sorted as Dickens, Charles. If the box is colored green, then the text matches the individual author's sort strings. If it is colored red, then the authors and this text do not match.
The error icon indicated that the current author sort does not match the current author. Not action is required if this is what you want."