What you are referring to applies to gold which is worn as jewelry and is exposed to the elements and has skin contact. Not to a gold dial cased in a 100 meters water resistant case. Besides 18ct gold shouldn’t tarnish to that extent in such a short period of time. Tarnish usually occurs in 14ct gold or less.Google, "can gold turn black". There are lots of reasons gold of less than 24K can turn black.
Nice to know it's nothing serious.I have good news. I had the watch serviced and I asked my watchmaker to take a look at the crow. He did and he just simply cleaned it. Apparently, it was simply slightly oxidised on the top. It looks like new right now. You can check my IG account: @analogueguys. Thanks everyone for help. No need to replace the dial.