What you are seeing is the moisture from the air inside of the unit condensing on the inside surface of the cold lens. Not much can be done about this. You can send it into Humminbird and if the Service Center still uses it they can add a sacrificial coating to the inside of the lens, but this only buys time before it starts again. Call ahead before doing this to make sure they still have and use the stuff.

Your best bet would be to store it in a location that has dry air. In a bag of rice would help but I prefer to use the moisture absorb packs like the reusable ones you can use is a gun safe. I have even heard that storing it in a cool location also helps or even moving it from a warm than cold than warm,… location will help move the moisture out of the air inside of the unit (with the moisture absorbent packs in a sealed container).