I was laughing with some people about how dickish this secret was. Maybe I took it too far. I'm sorry.
First of all, you can reset levels by either quitting to menu and reloading, or changing the difficulty, so it's not a one shot it you miss it.
There's also a 25% chance of the red slime having alternative dialogue which will give you a hint.
But if you really need me to just spoil it for you...
Changing the difficulty affects how strong gravity it. You need to be on easy difficulty to make the jump.