Scrub is a running gag between the Creatures, but mainly Nova and Kootra. It is a term used often by them. It is basically the equivalent of "noob", which is gamer talk for anyone that is new/sucks at a game. The term can also be used for other reasons, or just simply insulting others.
Nova is the Creature who most commonly uses this term, an example being in his Call of Duty: Ghosts Extinction series with Kootra he says "The aliens are really scrubby." and "There aliens are scrubs." Kootra also claimed the enemies in this mode were scrubs.