You are correct: In the USA "my dear + Name" is usually used when referring to close friends or family members.
My guess is that the fellow who addressed you in this manner was a.) trying to impress you with his informality, b.) trying to appear to be superior to you, or c.) hoping that you would reciprocate his informality when you replied. I don't believe it has anything to do with his passion for art, poetry, or other creative endeavors!
In any event, my advice would be to give this fellow the 'formal treatment' until you feel comfortable responding to his informality.