Learn English – How should one talk about multiple Greek letters in text?

I'm writing an academic paper in economics, and I'm using several different instantiations (I guess) of the Greek letter mu in several different mathematical equations. If I want to refer to all of them in my descriptive text, how should I do so? Should I write "mus"? Or "mu's"? Or should I insert the character itself, followed by just an "s"? Or followed by an " 's "?