It's a tricky question, but only because I was making it tricky by trying to bake it into the description for green slime.
The reality is that green slime is not really a monster. It's a trap, or part of a trap.
And when a DM plops some green slime down on the map, they should be able to use green slime however it best fits their ideas for encounter design, depending on what kind of slime it is.
Here are three kinds of slime.
The first kind of slime is always out in the open. Players always notice it.
You can see it covering the entire ceiling of the hallway, ready to drip down onto anything that walks below it.
With this kind of slime, the question is "how do we get past this hallway full of green slime?" That's what is tested.
The players must think of creative solutions to this problem. Rolling a wheel of cheese perhaps? Taking cover under a tower shield and simply running for it?
It's an interesting problem, and one that leads to creative problem solving. Good job, slime.
The second kind of slime requires players to look in a specific spot. If the players don't look in the right spot, they won't spot the slime.
With this kind of slime, the question is "how do we search our environment to avoid traps?" and that is what is tested.
If you fail at searching your environment carefully before advancing, you get a face full of slime as punishment for being bad at the game. Do you hear viscous dripping behind the door? Is the bottom of the fountain thick with green growth? Do you try to illuminate more of the mine shaft before you climb up it?
Also an interesting problem, and one that leads to thoughtful exploration. Good job, slime.
The third kind of slime will only be noticed by players if they roll well. Perhaps it is as random as a coin flip.
With this kind of slime, there is no initial question. You're gonna get slime on you, or you're not, and your choices have no effect on whether or not you get green slime on you or not.
So, you're gonna get slimed, and there's nothing you can do about it. The question becomes, "what do you do when you get slimed?" which is more interesting when there are multiple things going on at once, so you are forced to divert your attention between the slime and the other thing.
That's an interesting problem, and one that leads to tense moments. Good job, slime.
It's a brutal mechanic, that's for sure. But so are petrification and level drain, so there is precedent for brutal mechanics. I have a monster that spits green slime, which is a little more interesting, because then players have to choose between cleaning the slime off or dealing with this tubular cassowary. And since the players can learn of it's slime-spitting tendencies and perhaps get it to blow it's load onto a tower shield or something.
The fourth kind of slime is the bad kind of slime. It's the kind of slime that falls on you as soon as you walk into the dungeon and eats your armor.
You had no agency. You had no chance to detect it or bypass it. It's just a tax that is levied on you as you progress down the corridor. A shitty toll that you must pay in order to proceed.
It's the same if there is a Perception roll. Either you make your roll and avoid it, or you fail your roll and take damage. Again, there is no agency, no interesting choices to be made.
This is bad. Don't do it.
The Straw Man Speaks
SM: Hey, I thought of some situations that fall into multiple categories.
SM: You know, you could extrapolate this to all traps.
SM: So which one of these should I write down when I'm writing my Monster Manual entry on green slime. Which is the best kind of green slime?
AK: You haven't been paying attention. Don't write down any method of "how to spot green slime". You're only limiting its usage, and that's not the interesting part of green slime, anyway. The whole point of writing things down is to give us a hard copy to refer to, usually in shorthand. This gives you freedom to write up green slime traps however you want, which all have the same penalty for failure: you get green slime on you.
- Use obvious slime if the puzzle is "how do we get past it?".
- Use findable slime if the puzzle is "how do we search this environment for dangers?"
- And use luck-based/automatic slime if the puzzle is "how do we handle getting slimed?" and you are kind of a dick.