That's the designated helmet with the receiver in it that can communicate electronically with the transmitter of the coaching staff on the sideline. By rule only one helmet receiver is allowed per team.
It's on all the qb helmets this year. There is only one helmet with the radio system they have on the field at a time and I guess it helps them keep track of that.
edit: Oops just saw it was answered above.
I didn't even see Meach play. That's pretty bad, he's been disappointing.