direct from the source: "Octavia's character design is reused with a different color scheme in Luna Eclipsed. Her coat is yellow, her mane and tail are dark blue, and her eyes are light blue. She has the same treble clef cutie mark as Octavia, except that hers is light blue instead of pink. "
'The source' is a wikia, a fan-written site. Of course they would call her cutie mark a treble clef, because it resembles one. It still does NOT look exactly like one, and is rather an undefined variation of it.
I have drawn that variation. It is not a treble clef, it is not an ampersand, it is Octavia's cutie mark.