I think it would be very speculative for me to tell you what it is from some photos on the internet. Possibly some glue, or debris that became attached to some excess glue? A bit of corrosion? A watchmaker could try to remove it by gently touching a piece of Rodico. But that's unlikely to work unless it is very weakly attached. Anything more aggressive would be risky. And to be totally honest, it is barely visible to the naked eye. Although it's your right to do so, I personally think you are obsessing about something very minor.
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