I saw this on TV in the UK, probably in the late 80s. It was animated, and could have been a movie or TV show.
It featured a yellow slime, which could eat through anything. I don't recall if it was a chemical ooze or a 'grey goo' nanite swarm. I think they tried containing it in glass at one point, but it dissolved the glass.
Right at the end, the yellow slime has been defeated, but then one of the characters notices something.
"The yellow slime!"
"Don't worry, it's only mustard."
Although it could have been custard.