There might have been some water that somehow leaked into the bezel of your screen and when you turned it upside down it ran down your screen. So you might need to turn it off unplug it and get some screen cleaner and see if that will remove the droplet. If there is a thin plastic screen between the screen and the cover of the computer then there might be a drop between the case and the screen. It really depends on how the screen is manufactured. You might want to look on YouTube and see if someone has recorded your specific system being dismantled and determine if there might be water in your system or not.
First try cleaning the screen with screen cleaner, and a clean rag. Spray the rag then clean the screen with the rag.
If you still see the mark the try the take apart option after doing some removal research. Clean as before.