The milk simply looks like ordinary white milk from the real world. It comes from sea snails and it makes people fall asleep quicker. It is best served warm.
Role in episode
SpongeBob drinks snail milk to fall asleep faster after trying everything else in the book that he read. Within the first few seconds of drinking it, nothing happens. However, after SpongeBob screams about how he cannot go to sleep, the milk makes him go right to sleep.