The Parasite is most likely some form of very specialized slime mold.
Here are the reasons why it's an amoeba-like slime mold instead of a bacteria, a virus, or a fungus. It's a semi fluid mass that moves on its own by shifting its mass like an amoeba. It also has basic senses to find its food and feeds by dissolving its prey and absorbing it with its body mass. It also produces hardened stems for reproduction, just like real life slime molds. So other than being parasitic and having a ridiculous metabolic rate, it basically shows all the hallmarks of a slime mold.
