The core problem you may be seeing is because this is a Smart String application. Salt near the coast is a concern but rainwater is conductive everywhere (acid rain).
It is most likely Galvanic corrosion that you see. We are putting 12vdc inside connections not designed for it (the CAT5 jack and plug), and then add some mildly conductive liquid (acidic rain water and dew), so you will get galvanic corrosion.
Things that make it worse are: Dissimilar metals, ionic contaminants like salt, and higher voltages (and DC is much worse than AC).
Gold plating helps (Gold does NOT corrode, but it is only a few angstroms thick, so it wears off). CorrosionX should help. Keeping them totally dry would be best but isn't practical in this hobby.
If you already have corrosion, then gently clean, apply CorrosionX, and check it when weather permits.