I would consider mildew to form over a protracted period of at least a few days and be easily removed with a soft cloth and suitable sanitiser. The issue appears to be, at least from the photograph, more like corrosion, minute surface pitting. May I assume its gold plated? or is it brass?
In the future, do not leave the reed on the mouthpiece after playing it. Take the reed off and put it in a Reed Guard, wipe the mouthpiece dry and put it in a protective pouch in the case. You can play that mouthpiece for the rest of your life if you take care of it.
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