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This morning I noticed the smallest of hairs inside the crystal on my speedmaster.
I don’t think a scratch or defect in the lume could move between the second and third pictures, so I’m assuming the poster is correct and it’s a hair or fibre.
It’s probably very annoying now you’ve seen it, but it almost certainly won’t do any actual harm to the watch.
Better there than in your soup. I doubt that it will do any harm, but you can certainly have a watchmaker remove it when the watch is next serviced. The real question is how it got there in the first place.