Shadow Goo is part of Drawn to life's adventure mode.
Role in Gameplay Edit
Sometimes the player comes across Shadow Goo, covering a certain part of the level, and they must rub it off using their stylus. Enemies can grow out of them, so it is best to get rid of the goo, though it is entirely optional. This also increases the percent of the game, and affects how much money you get at the end. It can also hint that a Raposa, or page of The Book of life is near by.
The Shadow Goo is likely caused by the Shadow Creature enemies.