It's likely fibers from a cleaning cloth caught between the crystal and crystal gasket from when the crystal was installed. If the watch is under warranty, send it in to have them fix it. If not and it bothers you, seek out a watchmaker who has access to Omega parts show can remove the movement, then press out the crystal, and install it again with a new crystal gasket (these gaskets are one time use only items).
Cheers, Al
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