The Gutter Grime's Grasp (Jitter Jumpers, #38)

Fourteen-year-old Sydney is a "quiet observer," a girl who has perfected the art of being a shadow in her own life. But the town she calls home is hiding something "viscous" and "grimy." Beneath the streets, a monster is growing, fed by the "collective unspoken fears" and negative emotions of the people walking above. When the "Gutter Grime" begins to reach for her, Sydney realizes that ignoring the filth won't make it go away, it only makes it hungry.