Hi Rob, thanks for taking the time to look and ask. Essentially avoid the rock, or not. Mercer is a Meta character from “Do Androids dream of electric sheep” who is endlessly walking and getting rocks thrown at him by mysterious agents. He exists in a virtual reality machine that the characters in the book “link” up to. They do this to feel connected to everyone else who is “connected” at the same time. I kinda guess it’s a metaphysical game.