Ah that makes more sense. I was thinking he wanted to isolate the led driver, in fact he is trying to isolate the midi device from the Arduino noise wise which probably is a wise idea. I assume this mobius is running on the Mac and sends the midi information to the Arduino via the serial port? Does it need to be bidirectional (I'd assume so as data would seem to need to go both to and from the Arduino to the Mac if the foot switches are supposed to change things on the midi end). Something like this should be what you want (note the optoisolator needs to be high speed and the usual ones are not, the 6N138 mentioned here is high speed enough it appears. ) I'm somewhat surprised these aren't available on ebay but they don't seem to be. It would be worthwhile looking for a built one in the midi forums I expect if there isn't one it is available in 8 pin DIP (easy to use on breadboard, avoid smd ones) for a couple of bucks each. This link shows what you need to do you would need two of these to get both directions:
http://www.midiplayertools.com/Documents/MIDIspecCable.pdf
Peter