It is without a doubt a earpiece for communicating information and advice to rig the debate. It is much like a quarterback wearing a wireless coach-to-quarterback communications system inside their helmet. NFL offensive coordinators have literally had the ear of their quarterbacks since 1994. However, this should not be permitted in a presidential debate without the other debate participant having the same advantage. GOP Chair Reince Priebus as well as the Trump Team should be doing something about this foul on the debate field.
Trump would not need or use an earpiece. So I would not make the fairness argument. The better argument is why Hillary is so weak that she uses an earpiece and then doesn't disclose it (IF she is using an earpiece). It could also be a hearing aid.
I rescind part of my earlier comment. The fact we know that Trump would not need or use an earpiece is irrelevant. They don’t know that. Your fairness argument is valid.