The word is 箱庭 (hakoniwa), and I recommend looking it up on image search to see what that is. The literal translation would be "miniature garden", but I didn't want to use that phrase because without context, it wouldn't bring up the right image. Not that I think the image of Thorn gardening small flowers wouldn't be nice or whatever, but that's not the reason the word 箱庭 is used in the song.

The reason Thorn calls Mobius a "miniature garden" is that a miniature garden is an artificially cultivated space in a small, sealed-off area, so whatever I used needed to capture that. Like a miniature garden, Mobius consists of things that are seemingly natural but are deliberately tweaked to "grow" a certain way, and is maintained as a small, isolated paradise away from the rest of the world (reality). I initially considered breaking 箱庭 into its kanji components (a technique I use often when the components are closer to the intended nuance than the dictionary word) and going with "garden in a box", but I eventually settled on "pocket garden" because I felt that captured everything I needed. The correlation with "pocket universe" invokes the idea of a whole world condensed into a small, sealed-off area, and the impossibility of a garden fitting into a "pocket" invokes the idea of a paradise being artificially shrunken down and sealed away. Thorn uses the word in the sense of "I don't care if this is a false, fragile paradise, I still want it to last forever," so I wanted to emphasize that.