If you look at it from a legal standpoint, it probably is plagarism. Because Rap Monster used someone elses written words without his permission. If he noted the quote, he should note the source, si he can request permissions if he decides to use it later. That being said, I don't believe his intent was to plagarize. He is just a young man who probably didn't even think that using a mere phrase someone tweeted would be considered plagarism.