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

During the development of Windows 8, 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 Windows 7, 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.

source https://www.osnews.com/story/145213/the-placeholder-name-for-the-windows-8-experience-was-modern/