Burn marks on iMac Screen
This happens all the time. 

Condensation and fog like effects are almost always benign and transient. Shut down form15 minutes minimum and restart. 

The image persistence is somthing all LCD will eventually succumb to and some can show that even when brand new. Either power off entirely for hours or days or have an all white screen saver engage and prevent partial content from staying on screen static. 

https://en.wikipedia.org/wiki/Image_persistence