FYI: After I posted, A.J. alerted me that the sticker was in honor of his former teammate Nick Pasquale.
Last week against USC, Chase Rettig wore a circle sticker on the back of his helmet with the letters "NP" (see picture below). I took note of it at the time, but did not read an explanation after the game.
Then Saturday a series of players in other games wore circular stickers with the letters "APU." Their stickers were a protest about college athletes getting paid. Obviously the letters are different, so Rettig's sticker is likely unrelated, but I still don't know why he wore it. If anyone has any answers, email me or post it in the comments.