Slenderman

Slenderman is a fictional character referenced in The Fallen. He is most commonly portrayed as a very tall being with a pale, blank face, a suit and sometimes black tentacles. He has many powers and behaviours due to the amount of stories he is in, but most notably, he steals and kills children and makes people his proxies. He appears in Creepypasta and Slenderverse series (Like Marble Hornets and EverymanHYBRID).

"Appearance" in The Fallen
Ella mistakes Paul for the Slenderman around page 256, when she tells Maxie about the "Monster", which is really just Paul being sick, insane and hungry. Calling Paul the Slenderman may also be a reference to how Paul steals and eats Samira, much like the Slenderman in some interpretations, or how the Disease kind of makes Paul it's proxy. It could also be a slight reference to the Evan/Habit relationship that Paul and Boney-M have. (Evan and Habit are characters in a Slenderverse series called EverymanHYBRID. Habit is a demon that makes Evan do terrible things, very similar to what Boney-M does to Paul).