Sun Damage?

[Jenna87]

I just noticed this horrible mark on my cute polka dot dress! Is it sun damage? It's in the shape of the cut of the back. Is there anything I can do to fix it? I made the photos slightly darker to make the mark easier to see. Also, is this rayon?

 

[poppysvintageclothing]

It looks like a crepe fabric. Could be silk or rayon crepe, difficult to discern without handling it. It looks like it may just have faded from hanging, sadly that can happen.