The Evil Campaign
This room has an oozing green slime covering all the walls, Strangely however when the door was opened the slime did Not try to attack or anything, it just stayed on the walls.
A successfully monster lore and search check revealed that this was a symbiotic relationship, the slime gets feed the refuse of the palace.
The party then retraces their steps and comes to an unlocked door, opening it they see the a small 20×20 room with the walls covered with a green oozing slime. Except for the slime, the room appeared empty, they closed And locked the door.