This is the symbol of Freemasonry. If you look closely you'll see this symbol in the painting above along with the symbol that Nelly is photographed flashing.
Here Nelly uses the repetitive duality theme used by many MK/Masonic stars with his Sweat/Suit album. The double album is meant to showcase the two sides of Nelly. Very similar to Beyonce's new Sasha Fierce theme.
More celebrity duality/alter egos/split minds.