Let’s just come out and say this; if your watch begins to fog up inside of the screen then that is not a good sign. From your days growing up, you probably learned in science class that fog is made up of condensed water droplets. What happens is when the droplets are cooled to the point they can no longer hold the water vapor that is inside of them, fog appears. This means that if your watch is fogging up on the inside, there is most likely a sealing issue. Now, if the fog quickly dissipates after you clean off your screen then there is no issue. However, the issue becomes if the fogging effect does not go away.
Truthfully, the only thing you can do in this case is to either contact the manufacturer for further instruction or contact a local service center to see what they recommend.