How to clean the tanks and escape?Like most Hidden Challenges, the actions required are relatively straightforward. You will have to go to Sulrpy Swamp, in a factory north of the area (see map at the bottom of the article). Once inside it, you will have to bathe in the two tanks which are filled with Slurp so that it regains its blue that we know.
Once done, you will have to go north to approach the gnome waiting for you. He is just in front of the broken wall, on the right. When you are at his side, the challenge will be validated and the latter will take off.