...Could it be imagined as the paper on which the Codex is written? Paper is a lot like fabric. Right? Right??
Heartworm swallowing a piece of the fabric of reality is explicitly mentioned in this post
, summarywards, where it consumes part of the underlying structure of existence to become more like the background of all that is, thus explaining a point of concealment. That's always been Heartworm's teleportation mechanic: literally carving up reality to make a wound it could move through.(As an aside, it's also one of my most consistent long-term bits of character development. At first Heartworm had to laboriously chew through reality, but later it could carve holes using a tool. Eventually it could slice portals into existence quickly with its vehicle, and lately it's just been kicking reality so hard it rips like paper. An upcoming post is the first and only one in which Heartworm simply wills a wound in reality to open up. It's also been teleporting more as time goes on. So there's some subtle power progressions for y'all.)
The thing is that 'the fabric of reality' is a really common metaphor for 'the underlying structure of the existent universe' when it comes to describing both physics and metaphysical stuff. I'm sure most of us have used it here at some point, unthinking, to mean exactly that.
Unless the Fabric as you describe is something specifically other than that, then I imagine it's a Vowzrid phenomenon of the general concept. Heartworm might be interacting with a different layer of it somehow. It might simply not be attuned to the sentience, or there might be some mirroring effect in which its own weird AI soul reflects on the Fabric and makes it seem cold and lifeless from its perspective.