I have a CPL filter with what looks like smears on the back of the front glass (as far as I can tell the front of that glass is clean, and this thing rotates with the front glass).
Is it (or is it going to be) a problem? How did that get there(*)? How can I test the negative effect it could have on shots?
(*) wondering if is salt since I mostly use these filters when I'm near the sea.
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.
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.
However,
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.
Is it (or is it going to be) a problem? How can I test the negative effect it could have on shots?
Take some test shots and examine the results. Pay particular attention to backlit subjects with a lot of bright stray light in the scene or shining on the filter from just outside the frame. That's where you'll likely see the most noticeable effect.
How did that get there(*)?
(*) wondering if is salt since I mostly use these filters when I'm near the sea.
From the photo you included there's no real way for us to determine what it is nor how it got there. If it is, in fact, dried sea spray you'll need to disassemble the filter (not an easy thing to do without permanently damaging the mechanisms that hold it together) to clean it. Be sure to put everything back together facing the same direction.
