I'll vote for epithet, too - though the common usage of epithet is quite different. Or, just maybe, moniker.
I agreed with a previous poster that the phrase is more like a motto. I think for the phrase to be a colloquialism, its use is limited to a particular region. For instance, people outside Philladephia might refer to the famous sandwich as a Philly Cheese Steak, but a local may refer to the item simply as a steak sandwich. But I'm not sure ... How about an idiom? Would that work?
Ha! That's so funny I had one of those thunderbolt flashbacks last night. I thought the example I used was not fitting at all. But it made perfect sense to me at the time.