The "Grue" is a reference to the text-based game Zork that depicts it as a monster that dwells in the dark. When a player walks into an unloaded node the screen is filled with static and reads "You have been eaten by a grue, Press X to respawn". Upon pressing X, the player will teleport to the Spawn Tower, and it may take some time for the world to generate upon respawning. Alternatively, the player can opt to wait for the node that they are in to load. To keep track of nodes loading, open the map with the Left Stick.
To avoid walking out of loaded nodes, do not walk into areas with no blocks (if status markers off) or areas that have a green/blue 'rain' in place of a node.