TLDR: it's "bat" in German. As in that cute flying squirrel. If you care for more detail and where I got that name: continue reading.
The username "fldrmaus" was achieved by taking the German word for bat: "Fledermaus", making it lowercase and removing the e's.
The pronunciation is not supposed to be changed by that and is still [ˈfleːdɐˌmaʊ̯s] in IPA. You can listen to a sound sample on Wiktionary. The simplest way to describe it would be "fled - err - mouse".
In 2015 I picked the name "fldrmausmann" because I was listening to a lot of music by the German band "K.I.Z." and one of their songs was called, you guessed it, "Fledermausmann". You can listen to the song on their YouTube channel. You notice that back then there was a "mann" attached, meaning "man" in English and making the name a literal translation for "Batman" into German.
In 2020 I decided to drop the "mann" from the name because I didn't feel comfortable with shouting "HEY I'M A MAN I'M SO MANLY" into the world anymore.