The music provided in the Rocketium library is indeed royalty free and is attributed to the original musician as required at the start of every video. All of the music has been acquired legally (paid and unpaid) and is free to use for commercial purpose as well.
These notices are sent to ALL video uploaders who have used a piece of the artist's music in their videos to notify them that the artist is going to monetize the videos on your behalf. Unfortunately, YouTube can’t differentiate between your own videos and other people’s videos.
If you've received an email or a notification from YouTube regarding this, please click “OK, Got it” to resolve this.