As is de rigueur in these types of situations, the house is calling the split a "mutual" decision. Joel Palix, president of Clarins Fragrance Group (which owns Mugler) and director general of Mugler gave the following statement:
Nicola has accomplished our original mission of bringing his energy to the brand. With his talent for communication and understanding modern imagery and design, Nicola has been instrumental in attracting a new audience which is undeniably crucial for the future strategy of the house. He will be a historical part of the Mugler legacy and known as the force that catapulted us forward.
Which kind of sounds like they weren't intent on keeping him for the long term, but who knows?
Though Formichetti's collections have been met with mixed reviews, his designs became a bit more wearable and elegant in recent seasons; and with his final collection for the house--fall 2013--he seemed to really solidify his vision for the house in a sleek, yet very Mugler, way.
Clothing aside, his impact on the house can't be denied--mainly in terms of exposure--from collaborating with a couple of the most famous people in the world (Lady Gaga and, most recently, Kim Kardashian), to making Mugler one of the most prolific fashion houses on social media via Twitter, Tumblr, livestreaming, etc.
That said, he has always been a "globetrotting multi-tasker," as WWD calls him. With regular clients like Lady Gaga, Kim Kardashian and V magazine, he has a lot of other stuff going on and will likely continue to have just as strong a presence in the fashion world after his tenure at Mugler as he did during.
Formichetti Tweeted this afternoon: "its was a great 2 years at MUGLER. hope you all enjoyed our journey together. but now its time to move on!!"
As for Mugler, WWD reports that the house will embark on a new strategic plan “that builds on its highly successful legacy in fragrance and its new momentum in fashion.”