'Lime' build-up. Get a little mouthpiece stuffer and start swabbing out the neck with it after playing. To remove it right away you can use some white vinegar on a cloth that you twist into the neck. I keep 'Lime-Away' for similar uses plus removing tarnish from raw brass. You probably have the same stuff in your mouthpiece - same solution.
Swabbing out after playing will help. For now a soaking in vinegar is a good option, just remove the key first, but I wouldn't swab acid into the neck and just leave it there. You could soak the neck in a glass of pure white vinegar for 10 minutes at a time, brush out the neck, rinse and then inspect the neck. If there are still deposits, repeat the process until the deposits are gone. Citric acid works even better.
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