That is the the positive battery contact, and it has oxidation on it where it was touching the battery. If you look at the positive button of the used battery, you will see a black oxidation spot where it was touching the contact. On a microscopic level, arcing has been occurring between the two surfaces.
Arcing = Oxidation = Bad Connection = More Arcing = More Oxidation, and On and on.
This was why I bought the abrasive fiberglass pen. It shines up the contact really nice, but you will see oxidation again every time you change the battery. The solution is ANOTHER PEN !!!
Amazon.com: DeoxIT® GOLD Pen (NSN-6850-01-477-1478) 100% solution 6 mL: Electronics
You can get it for $14.50 delivered, click on "8 new from $9.49"
Clean the contact once with the fiberglass pen.
A thin dab of DeoxIT on the positive battery button when changing the battery, no more oxidation.
Use a Q-tip with alcohol on that wet looking spot, its probably nothing serious.