There's very little actual information about it, but it's supposedly a reality show co-directed by the members themselves. It's an answer to fans' long-time wishes to see a 9Muses reality show. The girls were reportedly thrilled with the idea and there's a promise of "actively showing individual charms of model-dols, as well as their quirkiness".

It is expected to be broadcast on the official 9M channel, first episode "Ppeppero day" comes out on 11th of November, which is a quirky enough holiday to begin with, so it's going to be something to look forward to for sure :P The official fancafé will also feature BTS cuts (1st episode BTS coming out on 19th), those will likely leak to YouTube as well.

Oddly, nothing addresses the 'Cast' part of the name, but the way I see it, that could possibly go two ways:

a) It could be a funny/serious program where girls also hold auditions to bring in new members. I personally don't think that is the case, because recruiting is handled by agencies with their own strict criteria. Members may have a say, but the initial filtering usually happens through agency itself. However, considering the recent shift of power in SE, anything could happen, really. But I believe if that was the case, it would have been mentioned in the available info, as it is a major part of the program

b) It could just be a not-so-fitting use of the word. In that case, it has nothing to do with casting new members, but it's supposed to mean "Featuring", basically saying it's a program about 9Muses, also co-directed by the members.

Feel free to add anything you want - new info, speculations etc. There really isn't much to go by at the moment.