A bass Guitarist I would venture to guess that a person who plays the bass guitar is called a "bassist". Calling a person who plays a bass guitar a "guitarist" would also be acceptable. Bass player Bassist Bass guitarist Guitarist is misleading
A "harmonicanist" is what a person who plays the harmonica is called.
A session musician; although the Beatles occasionally tried playing violins - who could resist trying out session musicians instruments - none of their efforts made it onto records.
Someone who plays the violin is a violinist.
You call someone who plays any instrument alone a "soloist."
Someone who plays the piano is called a pianist, but someone who plays the keyboard can be called a keyboardist.