Himihagi's true appearance is new, previously we only saw the statue and the shadow-form. It's the same white body as Pernida's so it's not exceptionally strange, but Ukitake becoming a shadow him calls to mind the other black-white dualisms through the series.

I think this is occuring in Ukitake's inner world. Mimihagi is the source of the other half of his dual zanpakuto, but I'm pretty sure it's taking the position of the true essence of that half here. Yhwach is similar to the soul king, so similar to how Yhwach is the essence of half of Ichigo's own.