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From your description, it sure sounds like calcium (with a little algae for the green). Calcium should react to acid and fizz (like Ben said).
I've got a thought, but I'm not sure it would be safe for the liner, so I'm leery of mentioning it. Replacing the liner would take this issue out of the picture, but I don't know if your finances allow for that. If they do, you can try what I'm thinking of and, if it wrecks the liner, you were ready to get one anyway.
I'm sorry that this post is pretty much 'non helpful'
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