Coynmond I don't believe it's an acronym.
There is a connection to "A rolling stone gathers no moss" (e.g. with the package format also being called .stone, the build tool boulder, etc.), but I don't know if that was the original idea.
edit: Okay, it seems to have been the original idea:
Moss is so-called as we’re a rolling release, a rolling stone gathers no moss.
From: https://aerynos.com/blog/2020/09/07/stage3-complete/#announcing-moss
edit: outdated speculation:
Perhaps "Irish moss" also played into it, what with Ikey often putting references to Ireland or the Irish language into his project names.
Funnily enough that would also tie in with the rock/stone/boulder aspect again:
Chondrus crispus—commonly called Irish moss or carrageenan moss (Irish carraigÃn, "little rock")—is a species of red algae
