The Slime (スライム) effect turns Sabitsuki into a green hunk of slime, as its name implies. Pressing the action button will make her sink down into a puddle of goop and back to normal again. It can be found in a section of the Rusty Pier by interacting with some slime of similar color at the end of one of the pier sections.
Go right in the Nexus, then follow the directions on Red and Blue World page to find it. Once in the Hospital, go to the direction with the most blood. The area you want to end up at is completely dark with red lines and horrifying music but it's good to mention the area before it is another hospital area (just go through the door). Once you are there, follow the red lines up, then make a right to the last path that goes up (there are three but you're aiming for the last one). At the very top, there is a white cylinder with blood dripping from its teeth. Go in and you'll be greeted with a giant monster. To the right, there should be an entrance to the sewers.
Go left to the next screen, then go up the incline nearest you on the screen. There should be an NPC there that you need to go around. Go right two screens then enter up the latter where a giant slime thing is. Go right and you'll enter the Rusty Pier. From here, follow these directions (most of these will be as far as you can go):
right, down, right, up, right, down, then left. There should be a pile of slime!
- Contrary to popular belief, this effect is not necessary to witness the White Room event.