What would you call a person who stalks other's monitor? I have a colleague at work who looks at my monitor whenever he passes by. How would I describe such person?
If he has no business looking at your monitor (that is, it is not part of his job to monitor what you're doing on company time), then he is a busybody.
That is defined as:
A person who meddles or pries into the affairs of others.
Someone who interferes with others; one who is nosy, intrusive or meddlesome.
A term to describe this type of activity is:
...the act of obtaining personal or private information through direct observation. Shoulder surfing involves looking over a person's shoulder to gather pertinent information while the victim is oblivious. This is especially effective in crowded places where a person uses a computer, smartphone or ATM.
A slang term used in the the gaming community to describe cheating by looking at another user´s screen is:
screen peeking (sometimes cheating, watching, surfing, hopping, etc)
but it usually is used when video gamers are on a split screen engaged in "shooter" type activity.
So you could call this person a shoulder surfer, or a screen peeker.
Someone who rubber necks. Or a rubbernecker, basically someone who will go to great lengths to look at something which does not concern them ie contorting their necks to see around obstacles.