The Green Fluid IS Explained, DOES Have a Resolution
It's amazing to me how many people come to this page and claim the green fluid "just disappears" has "no resolution" or is "never explained." You must all be doing something else while you're watching the movie, because the green fluid has a full and complete arc:
The green fluid is the Prince of Darkness. We know this because we know it is conscious and we watch the researchers decipher the codes. We also learn through what they discover, that it wants to bring its father ("Anti-God") back into the world from another "dark side" dimension.
The green fluid takes control of people for its own use. We see this when it takes control of several researchers, being passed by mouth, most notably Lisa and Susan.
Lisa and Susan haul the big tube containing the stuff up to the dorm rooms. We know this... BECAUSE WE SEE IT HAPPEN.
The green fluid then exits the tube and pools on the ceiling, above the sleeping Kelly, who has previously been marked. We know this... BECAUSE WE SEE IT HAPPEN.
The green fluid then enters Kelly through her mouth and swells her belly. Over the course of the early morning, it merges with her, causing her belly to lose its size, but altering her physically. We know this... BECAUSE WE SEE IT HAPPEN.
The being that awakes is no longer Kelly. We know this because it is evil, doesn't remember anybody as a friend, and calls the evil on the Other Side "Father." Again... WE SEE THIS HAPPEN.
After Kelly/Prince of Darkness is pushed through the interdimensional portal (the mirror) and the priest breaks the mirror, both Kelly AND THE GREEN FLUID THAT HAS MERGED WITH HER are trapped on the other side. It follows logically, right? The Green Fluid has BECOME KELLY, so if she is pushed through the portal, THEN IT IS TOO.
The connection between the Kelly Monster/Prince of Darkness/Green Fluid broken, the tiny remnants of it inside the controlled people becomes vaporous and floats away. We know this... BECAUSE WE SEE IT HAPPEN.
Now, I am not sure what cut of the film you people are looking at. Maybe we have all become so used to movies that explain through exposition all throughout, what is happening, that we cannot see an old style film that actually SHOWS YOU INSTEAD OF TELLING YOU. Maybe Carpenter was shortsighted in not foreseeing a time wherein viewers would not be able to determine for themselves what they are seeing. If he had, he might have let the Parker Jameson character narrate every single thing going on so that a future audience would be able to comprehend.
I don't know. I always thought actually SEEING A THING pretty much informed you what you needed to know about it.