I am sorry this reply has taken so long but it was only recently assigned to me. I have looked over your photographs very carefully and am pretty sure this is not some kind of parasite attacking the katydid. When I look closely at the dark green "blob" I can see little white wiggly lines covering the surface. These are very characteristic of the trachea or air tubes that supply oxygen throughout the insect's body, including the internal organs. Only insects and a few other arthropods have these kinds of trachea. They would not be present if it was some kind of leech or worm. And if this was some kind of parasitic insect larva, then the tubes would be internal and also not visible, and parasitoid larvae are not green, they are whitish. While some chrysomelid beetle larvae are dark colored, there is no evidence of the distinct head capsule and legs that these larvae possess, and I am not aware of any of them that are predatory in this nature. Therefore I am fairly certain that what you are looking at is a section of the stomach or crop of this insect that has somehow become everted out through a tear in the thin membrane that joins the head to the prothorax of this insect. The dark green is due to it being full of plant matter. And one of the photos shows the head displaced slightly off to one side with a thin gap between the head and prothorax cause by the internal loop of the gut poking through. There are also a few drops of dark green fluid on the head capsule that is very similar to the dark green "spit" that these crickets can regurgitate from their crop/stomach as a defense. I suspect those green drops came out of the everted part of the stomach. I am not sure how that could have happened, but it was most likely due to some kind of trauma that squeezed the body of the cricket and forced the loop outward through the weak spot. These types of crickets can also be parasitized inside by large worms that we call horsehair worms (Phylum Nematomorpha). I have never heard of it happening, but it might be possible that if this cricket was harboring one or more of these worms that it took up so much room inside, that the pressure on the organs forced the stomach to evert through a weak spot in the neck. Regardless of how it happened, this cricket is doomed. The constriction of its neck will prevent any food from passing through the gut and it will starve even if it doesn't succumb directly from the injury.