As long as you weren't showing movies or youtube video's at the time it shouldn't really be a problem. If you were showing a movie or a youtube video at the time for Educational perposes I suggest talking to him with a co-teacher and explaining why you were showing a movie/video.
In general I've found most if not all Principals Vice Principals dislike it when the waygook teacher shows video's or movies.
Another problem may arise with some student centered activities like Running dictation that could easily be misinterpreted as losing control. If he observed something like that I personally would have a talk with him with a Korean co-teacher .
This is some great advice. A lot of what have become common class activities like running dictation, telephone, or other similarly active games are way outside of the norm compared to other classes students sit through. I have found that some principals expect the native teacher's class to be more active than the classes going on in adjoining rooms while others are simply not prepared for the different teaching style.
For instance, my first principal was very concerned about English education and liked to see some new things - active games, projects, use of technology, etc. - and often (once or twice a week) stopped in to see what the students were up to. This never felt awkward to me because I felt like his attitude and intentions were clear.
One principal I have now has expressed some issues with how much my coteacher participates in our lessons. He thinks the classes should be pretty subdued and mostly English-only. Unfortunately, his office is just down the hall from the classes I teach, so he has asked a couple of times what we were doing when the class was being a little noisier than usual.
There are many, fairly different generations working together in a public school here. Besides individuals differing in opinions, every generation had a very different schooling experience that weighs on their current attitudes about how students should learn.
So, I second fishhead's advice - give some explanation up the hierarchy if you are doing something a little different, just in case. Though, many different explanations for the sudden observations are likely. Besides, giving some explanation will show that you noticed your principal's interest and should reflect positively on you.