The placeholder name for the Windows 8 experience was “modern”

Raymond Chen shares some history regarding Windows 8's development:

During the development of Windows8, we needed a name for that thing we're creating." Not being a particularly clever bunch when it comes to code names, we just called it the modern experience," to distinguish it from what we had in Windows7, which was called the classic experience."

And then, as Microspeak demands, we started abbreviating like mad.

Raymond Chen

Basically, they added mo" for modern" in front of everything, so the Metro shell became MoSh", the Settings application MoSet", and so on. And yes, the code name for the Photos application was exactly what it sounds like.