A family of Microsoft word processing software products for creating web, email, and print documents.
Yes, it's possible, but that also means using a macro-enabled document so you can use a macro to toggle the highlighting. There is, however, no way you can force the document's users to allow the macro to run, which means the highlighting won't necessarily get removed once they update the content control.
Subject to the above, such macros might be coded as:
Private Sub Document_ContentControlOnExit(ByVal CCtrl As ContentControl, Cancel As Boolean)
With CCtrl
If .Range.Text = .PlaceholderText Then
.Range.HighlightColorIndex = wdYellow
Else
.Range.HighlightColorIndex = wdNoHighlight
End If
End With
End Sub