There is no particular phase to describe it as per my knowledge. It's usually known as Triangle Player.
e.g.:
John Deacon of the rock group Queen assumed the role of a triangle player during the band's live performances.
It matters which brass instrument. Someone with a Trombone is a trombonist, someone with a tuba is a tubist, and someone who plays a Trumpet is a trumpeter. It's all very complicated.
No, that is why they call it a beginner band.
An alphornist is a person who plays the alphorn - a long, curved horn used by mountain-dwellers in the alps, originally to call cattle but now as a musical instrument.
A "harmonicanist" is what a person who plays the harmonica is called.
Call and response is where either an instrument plays a melody and the singer echoes it or vice versa.
A person who plays Scrabble is called a Scrabbler. (unofficially)
"Soloist"
A drummer or percussionist plays the drums.
harpist
An organist
a cymbalist
Guitarist