Clifford isn't really amused by the fact Sena doesn't remember the name of the quarterback of the team he was supposed to be playing in the finals, but considering that he's clearly wearing his home uniform and that Sena's an idiot, Clifford's going to let it slide. This time.
"Since I'm such a nice guy," translation: you owe me for this big, "I'm Clifford D. Lewis and I'm your 'ex-teammate', Eyeshield-kun." It's very clear Clifford is intending to capitalize on this.