What if a receiving player accidentally touches a punted football?

Answer 1

As soon as the ball is touched by a member of the receiving team, the ball is live and can be recovered by either team. (If the receiving team does not touch the ball, then a player from the punting team can only down the ball, not recover it.)

Answer 2

The ref can pick up the flag after the play, and usually announces that there was no penalty on the play.