Hello, You're Dead was the title of one of the horror movies owned by Rodrick. The movie's plot involved a disembodied Muddy Hand going around killing people, and the last person who sees it is always the next victim. When Greg and Rowley watched the movie during a sleepover at Greg's house, Rowley had his eyes and ears covered the whole movie while Greg saw it and later described it to Rowley.
Greg felt that special effects in the movie were really cheesy and wasn't even scared until the very end of the movie--at the end of the movie, after the Muddy Hand kills its last victim, it crawls towards the screen, making it seem like the viewer(s) is/are the next victim/s, and that's what ultimately scares Greg. Greg and Rowley (especially the former) took drastic measures to avoid the (obviously impossible) scenario as much as possible.