twitter(https://twitter.com/kawanove) のものから、少々加筆しました
ウエディングの定番、クリオネ衣装です

Since it's twitter(twitter.com/kawanove)'s property, I've only touched it up a very tiny bit.
It's a garment based on Sea Angels, a wedding classic.

See here for more info on Sea Angels / Clionidae.