adding a diode may not help. it will have a voltage drop across it of about half a volt for your typical silicon diode. for a 5V power, that's already a massive 10% lost.
Also, the PC and the monitor's 5V don't have the same ground reference so if you only plugged the 5V wire it can have funny results, and the Octo may not like being powered from an output (without the diode that is).
If you connect both the 5V and ground, you then also have two different grounds.. that the data line is referenced to and i imagine that can also make the lighting glitch.
if the 5m cable is causing issues i would honestly consider not using the octo to power the monitor lighting
A Farbwerk Nano is perfect for external use, and you can connect it directly to your monitor's USB, or to a powered USB hub you may be using on your desk already.
If said USB connector has enough juice to go beyond the 500ma a USB port usually provides, you can allow in aquasuite the FW nano to pull up to 2A and get them strings pretty bright.
That's what i use for my desk edge lighting, conected under the desk on a powered hub. works a treat and super bright if i want it (using 6 rgbpx strips, so 90 leds)