It has to be an Artificial Watermelon. A real one wouldn't keep this long without being invaded by Fruit Flies.
An uncut watermelon can be stored for at least two weeks in the refrigerator, but once cut, it's only good for 3 - 5 days in the refrigerator in an airtight container. This one was not refrigerated. It's either Artificial or when they cut the thing open it will smell like a Dead Corpse that hasn't been discovered for a Month.
In my opinion scrap it and go to the store and buy a fresh one.