What do you call someone who is conducting surveillance

I initially assumed surveillor, but apparently that isn't a word. Is there a word for such a person that derives from surveillance? Meaning it starts with the root "survei-"

His job is to conduct surveillance at the mall. He is a _____.

It's surveillant (as mentioned in the comments) or spy.

surveillant: One that exercises surveillance.

(AHD)

spy: One who secretly keeps watch on another or others.

(AHD)

Even though it doesn't start with "surveil-," I will go with [static] security officer/guard and loss prevention officer or specialist or detective.

Careers in Mall Security: Job Options and Requirements

Static Positions

Static mall security positions are primarily surveillance positions that require the officer to monitor the property from one location, usually by watching video feeds from security cameras. Static officers then communicate their findings to mobile officers, who investigate and address the situation. Static security officers may also be called loss prevention specialists or loss prevention detectives; mall or store management may hire these professionals to develop strategies to minimize loss by theft, and train other security officers in applying those strategies. Study.com

Watchdog is used at times, for instance UN watchdog is keeping an eye on facilities.

Another example here: http://www.theguardian.com/world/2015/nov/06/un-watchdog-confirms-mustard-gas-attack-in-syria