1.) Save map > Restart FH > Try Again
2.) Save Map > Delete Some Things > Export
3.) Save Map > Clear FH > Try reinstalling
You might have a particle attached to a mesh as Jay has mentioned, where you see this mist, try clicking on the meshes around it and checking for any attached particles. Or, you can try saving the map again then exporting the file. Also, you can temporarily clear your exports(just back up the inside files!) and try it after wards, I had the same issue and this worked for me.
Best wishes~
~ Whisper