Alfons Maria Mucha, known internationally as Alphonse Mucha, was a Czech painter, illustrator and graphic artist, living in Paris during the Art Nouveau period, best known for his distinctly stylized and decorative theatrical posters, particularly those of Sarah Bernhardt. He produced illustrations, advertisements, decorative panels, and designs, which became among the best-known images of the period.
https://en.wikipedia.org/wiki/Alphonse_Mucha
Mucha more: https://artvee.com/artist/alphonse-mucha/
I think it’s indeed a dress we’re seeing, which seems to have an interesting under-garment (salmon) component, as well as a white & yellow-green ‘over-garment’ component.
Meanwhile, the rump/thigh area color seems to be a surprisingly close match in color to her upper arm color, hence creating an optical illusion of sorts.
Anyway, I’m not aware that Mucha did much in the way of nudes or near-nudes, but that’s certainly one of the most unique dress arrangements I’ve ever seen… whether it was based on reality or not(!)