Formal Title for Someone with a Masters Degree?
In the United States there's no formal pre-nominal title held by individuals holding a Master's degree that I know of, so it's very unlike Dr. Xxxx that one holding a Ph.D., etc. might be addressed. In text, you would address such a person with a post-nominal suffix like M.A., MBA, etc. I'm sure the logic is different among specific types of degree, where Esq. for Esquire is a post-nominal suffix for those in the legal profession in the US. To give you an example, though: If I hold an M.Sc., you might refer to me as "Kendall, M.Sc." if you wanted to address me in an email or letter, but I would realistically expect Mr. Kendall or what have you face-to-face.

I should also add that courtesy titles like Esq. or Mr., and academic titles don't get used at the same time. Kendall, Esq. M.Sc. would be wrong, as would Mr. Kendall, M.Sc.!