This is a movie that didn’t come out until the early 90s, since I remember watching it as a child. There is one scene that sticks out, unfortunately all other details are lost on me.
In the movie, a group of adventures are travelling in a pinkish tunnel. Behind them, a group of people are chasing them.
As the chase goes on, the pursuers gradually are covered in some kind of moss-like goo, almost like seaweed. The dark slime becomes bigger and bigger, until each person is round and has grown in both height and width to just fit through the tunnel.
The screen will switch between the chasers (documenting their transformation) and the chased. Eventually the people running ahead (still people) find a ladder and climb up to escape.
I’m not sure if this is a children’s movie or a proper horror film, or maybe just a dark comedy film. All I know is that it has made a permanent mark in my memory and as a child I was terrified of tunnels!!