That is most probably fungus. That comes when the equipment is stored in a humid environment. Since you said you live by the sea, I think that's the issue. I do not believe it is salt since salt would create scratches when you twist the ring. Since I do not see any spiral scratches, I do not think it is salt. I think it is more likely mold/fungus/mildew or whatever the technical name for it is. Ask a biologist. I just call it fungus.
I recommend the following course of action.
First take some pictures at different lighting conditions and using different aperture values in each lighting condition and at different focus points ranging from infinity to min-focus.
See if the smears are visible in images.
- If not visible, ignore it.
- If visible, then do the following:
Take it to a camera repair shop and ask them how much they would charge for it. If it is reasonable, let them do it. If it costs as much as a new filter, just buy a new filter if you have the money for it.
If you are short on funds or need the funds elsewhere more important, see if you can live with the image quality. If the streaks are visible only during a very particular type of shooting, simply avoid that type of shot.
- if they are visible in all images,
- and the shop is too expensive
- and you cannot afford the repair or replacement,
Only then try self/DIY repair.
You can attempt to clean it after dismantling it.
AFAIK, you need some specialist tools like a circlip pliers and a ring/barrel screwdriver(I don't even know what that tool is called. I have seen it being used in camera lens shops to open them up.)
There is a very high chance that you could break it.