The stain left by silent tears flowing through the soul that cleansed the heart of the watch’s previous owner…..or, more likely, as @STANDY says, BBQ sauce.
Kidding aside. Could be a defect in the lacquer or paint on the dial.
Omega may have replacement dials, but will need movement caliber and case reference. Will also need to find a watchmaker with a Omega parts account to order one if available
An indentation from below that occurred during a prior battery change? The stain could be a paint touch up.
It's just a guess based on the fuzzy pics. If it's this, then it won't get bigger. Moisture that causes mold will get worse. The rest of the watch looks free of moisture damage. A leaking battery could cause damage to grow. Not possible to say from here.