Ruby referred to Johnnie as a British musician /the violin-playing guy from the subway, and the movie frames him as a brooding street artist rather than a polished student.

In context

She meets him as a stranger at first, then later learns more about him after they keep crossing paths in the city. The story describes Johnnie as a busking violinist in the New York subway, while Ruby is the classical dancer on scholarship.

What that means

If you mean the exact wording Ruby used when talking to Jazzie, the available information doesn’t show a precise quote from that scene. From the film descriptions, though, Ruby seems to identify him by his role and personality rather than a nickname.

TL;DR: Ruby appears to describe Johnnie to Jazzie as the British violinist/musician from the subway.