When I was a kid, somewhere in the late eighties or the start of the nineties, I got a glimpse of something my older brother was watching. It was on TV, not a rented video, so it probably was a series or an older movie.
Four (?) astronauts went to space. There was an incident and only one returned to earth. The others disappeared. There were four clear puddles, like jello.
The surviving astronaut goes into the world and enters an elevator. He reduces the other person in the elevator to a clear puddle by looking at him/her. That's how you find out the puddles in the spaceship were all the astronauts and the survivor is the alien.
This movie/series episode gave me nightmares for some time, I'd like to see it in full.