It was basically impossible to follow Nero for the others as he moved too quickly. Fortunately, his freezing body left a trail of frost wherever it ran, his icy footprints easily visible within the underground tunnel.
There were also signs of frost on the walls where Nero had grabbed mushrooms and just stuffed them down his throat. Such a thing was actually incredibly dangerous for they could be toxic, yet Nero didn't have much of a choice.
He used what little knowledge he had of foraging from survival class to judge what could be eaten and couldn't, and swallowed whatever seemed even remotely edible. That included any moss he found on the walls.
What Nero did not do, however, was attack any of the small animals in the caves. While the meat on their bodies would serve as a lot more sustenance than moss, hunger had not made him foolish enough to provoke the only animals that didn't want to kill him.
