Mushishi Special: Hihamukage is the continuation of the the original series of Mushishi, serving as an interlude between the original and later seasons.
It follows Ginko as he tries to solve a disastrous event where a mushi blocks the sun. It came out several years after the original, surprising many.
The art style remains true to the original and the soundtrack is as astoundingly beautiful as ever.
As the original show followed a very episodic structure you can watch this without much trouble, even if you didn't watch the original series, but some of the characters lose some background and introduction.
I would say that anyone who liked the show will most likely like this as well.
Enjoy!