I just wanted to know why the nickname "GreenMist" and "Green Mist" is banned.
The makers of X and Y have implemented a blacklisting system to the game to prevent inappropriate names appearing on GTS trades as did happen in Black and White. Unfortunately as Pokemon is an international game they have to make the censor work over 100s of languages. This means that some words may contain words inside them that may not be offenseive in your language but in another language it could be.
I couldn't find what was wrong with "GreenMist" but there's an example below.
You would think the name 'Violet' would be fine but in French 'Viol' means rape so the censor picks up the first 4 letters as offensive content and the name is blacklist.
It's a faulty system but I feel that is what you have encountered.