I think the answer lies in whatever your life has resembled over the past several months, so someone else can help you interpret the symbolism, but I found it interesting that I instantly thought "huh. he doesn't belong there" as I started reading your post (before I got to the part where you said as much yourself.) Parts of our brain remain active during sleep so the emotional salience of these jarring visuals combined with the hormones your body releases during a nightmare could be why you can remember it as vividly and have lingering anxiety since that night. For what it's worth, it sounds like you were just a misplaced observer in this dreamscape and there's little to gain by analyzing the details. Was it hard to get the story out in writing?