The Little Mushroom

This is mentioned by Wikipedia, which refers to The Life of Schubert, by Christopher H. Gibbs (2000): "Schubert, who was only a little more than five feet tall, was nicknamed 'Schwammerl' by his friends, which Gibbs describes as translating to 'Tubby' or 'Little Mushroom'. 'Schwamm' is German (in the Austrian and Bavarian dialects) for mushroom; the ending '–erl' makes it a diminutive."

So he was known by this nickname to his friends, and not by the public at large. And it was almost certainly used only in German.

These are small distinctions, but that doesn't mean they're not worth mentioning.