That's a hydrogen vent fin disposing of GH2 (gaseous hydrogen) that can outgas into the payload fairing from the Centaur upper stage's LH2 (liquid hydrogen) tank that is stored in a balloon tank at the top of the upper stage. From History of the Titan Centaur Launch Vehicle (PDF):
This liquid hydrogen leakage caused explosion of the launch vehicle during the first Atlas-Centaur launch in May 8, 1962. According to Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958-2002 (PDF), The Failure of F-1, the First Atlas-Centaur Launch (starting on page 63):
Testing at Lewis not only provided independent verification of
contractor performance, but also contributed new solutions to
problems. The loss of F-1 had occurred within a tenth of a second
after hydrogen gas had been vented from an opening at the top of the
vehicle. A test program in the 8-by-6-foot supersonic wind tunnel at
Lewis revealed that the hydrogen-venting system posed a fire hazard
during flight. The solution was to design a vent fin or snout on the
nose fairing that extended about 50 inches from the tank, just far
enough away to keep the hydrogen gas from igniting along the hot
surface of the vehicle.
This GH2 vent fin is not unique to the 5 meter diameter payload fairing used during AFSPC-5 launch, for example, here's a video of SBIRS GEO-2 launch on Atlas V 401 with a 4.2 m Ø fairing:
It is shown in Atlas V Launch Services User's Guide (PDF) in the schematic for the 4.2 m Ø fairing (page 267):
And identified in text for the 5 m Ø fairing (page 283):
In this Universe Today article, on the first photograph (I can't reproduce it here because it's copyrighted), you can see that vent fin connected with umbilical to the launch tower, where it would be purged with nitrogen provided through the air conditioning (A/C) inlet (seen on photograph as the topmost duct) and vented out the vent fin at the bottom of the Centaur upper stage's forward adapter, just above the CFLR (Centaur Forward Load Reactor) deck.