I know little or nothing about MIDI so please bear with me.
How does the Arduino know what port and speed to use when communication with Hairless ?
Doesn't MIDI use some odd baud rate ?
Do you need to use the MIDI library on the Arduino if Hairless is just expecting a formatted message over serial ?
This page looks like it might help you, at least with sending MIDI messages to Hairless http://www.instructables.com/id/Send-and-Receive-MIDI-with-Arduino/?ALLSTEPS