So I decided to follow around some Elk that spawned naturally on the surface amidst the muck.
They followed the normal pattern but I managed to catch a glimpse of one mid-death. This elk is "Shaken to its core!", which I assume is a fear token. It has sustained bruises on every imaginable part of its body. Its status screen lists yellow for most parts, and light blue for one of its eyes. It's also blinking "X" for Winded, which I assume means it's losing respiratory function.
As I toggle between pauses to check its status, it's not sustaining damage on any more parts, but it is gaining more yellow. Its left eye, for instance, is now also yellow instead of light blue. After it dies, it's coated in stinking muck and pus.
The rest of the herd collapsed all at once. They all have similar injuries. One sustained spinal injuries. Another had kidney injuries. They all begin to get pus as coatings before they die. They die at different times, usually very quickly after they begin to become winded. The only parts of the body that don't become injured are their hoofs, which is probably because of the material. I also have Giant Beetles because of my mod, and I believe they are the only things to experience contact with the muck and survive. I'm pretty sure that's thanks to their exoskeleton.
So, conclusion: The skies above my fort bleed ooze that melts flesh.
Edit: 100% causalities first migration wave. There is no salvation from this red death.