Based on picture above, this looks like exterior hazing of the headlight assembly (clear plastic piece) not moisture/fogging. This is caused mostly by ultraviolet light (yellowing) and road grime wear. You can clean the grime off, but the yellowing requires more work.
The yellowing is inevitable after ~3 ->7 years depending on if car is garaged or not, since headlights are made of plastic/polycarbonate. This can be removed by wet sanding the surface of the headlight with increasing fine sand paper and then applying a UV protectant liquid. I've done this on previous vehicles and it is a time consuming/labor intensive/tedious process to do right (~1 hour per headlight). Don't use the quick fix headlight restoration liquids. They are snake oil and do not work. You have to remove the fine layer of plastic that has yellowed because of UV light. A good auto detailer can do this work to restore the headlight assembly. Much less expensive than replacing the headlight assembly.
I put automotive film on my headlights covers to protect them. This film is amazing stuff (I am considering having my body filmed

). If/when the film becomes discolored/damaged I can simply remove the film and have brand new looking headlight assemblies. The film is clear/not noticeable and does not appear to affect headlight brightness.