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Possible causes are fungus, which grows in moist air, or that at least 1 element uses an unstable or radioactive type of glass - some do change colour or go cloudy over long periods of time.
If it comes clean, consider cleaning the inside of the metalwork too as whatever it was will be coating that. Then make sure its thoroughly dry before re-assembling (several hot days) and even then you may need to use desiccant to draw the moisture out of the air inside it.
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