EDIT: Answering directly your question - the method that best follows your request is to use headphones instead of Main L/R outputs in order to isolate the hum/buzz from your microphone. You'll still hear the buzz in your ears - but the audience won't. I eventually get to that suggestion below - just copying it up here.

Check your cable - maybe it will help

Some USB cables have filters on them. Do yours have this?

Example:
https://www.amazon.com/Tripp-Lite-Hi-Speed-Ferrite-U023-003/dp/B008VOPCGY

I would imagine Yamaha's internal testing probably tests with better cables. It would have been worthwhile to ship with a USB cable if this made a difference in the user experience.

Although that's not really the end-all. Ferrite isolation may not solve the issue. In fact, it often doesn't.

You can change the ground loop by paying attention to what the AC power source is for your keyboard vs. the AC power source for your iMac. Is your iMac using a power adapter - or battery powered at the time? That may make a difference. Plugging both (Mac + Keyboard) to a common power strip may help.

That may or may not help.

Ground loop fix - DIY not recommended

The pathway of of the ground loop is between the keyboard and your computer and each device's own ground reference. If you cut the ground link between the computer and your keyboard - you sever the path of the ground loop and then each device is fully isolated. Cutting this path out can have a negative impact on either impedance or noise immunity on your digital path. Although because USB is differential - it can mostly tolerate this sort of thing.

One DIY approach would be to physically cut the power/gnd wires inside your USB cable. If you have a ton of cables lying around - you may be able to tolerate getting a few wrong (cutting the wrong wires). Since your keyboard supplies its own USB power - there shouldn't be a need for it to be supplied by your laptop. ALTHOUGH - this is not really a given and still, you're not necessarily isolating all of the sources of noise doing this "hack".

I'm not sure if, technically, MODX does anything with the power rails on USB. If it, for example, uses this to detect a cable is plugged in. I wouldn't think so - I would think it would use USB handshaking to determine that and not bother with the power. I don't have a MODX to test this out and I'm not looking at MODX schematics to determine it either.

If you're going to go DIY - make sure you don't have loose ends exposed and shorting with each other (GND to Power) or shorted with anything else (chassis, shielding, etc). Electrical tape to tidy this up or shrink wrap can help. And DIY should assume you have proficiency enough not to cause damage in the process. I would say don't do it this way if you have any concerns. Just buy off-the-shelf adapters.

Ground loop fix - off-the-shelf adapters (not officially recommended - but better)

Rather than DIY - you can purchase an adapter that does this for you. There are likely lots of choices.

Here's an example:

https://www.youtube.com/watch?v=go7mmyaj25I

Note that the above device is not particularly rated for the audio interface of MODX which I believe is high-speed USB 2.0 (faster than this device supports). See below for that warning.

https://www.youtube.com/watch?v=GbNyINuo-Uw

The above video alludes to adapters that are much more expensive for isolation purposes. I imagine they, the more expensive isolators, are also higher speed and would support MODX's throughput. I don't have specific links to those devices. Also - due to the possible latency hit and also lack of testing by Yamaha - you're going to be "on your own" doing stuff like this. Yamaha recommends nothing but a single "high quality" USB cable connection between the MODX and your computer. And nothing else.

There are other choices of audio routing that can use other equipment instead. This would involve still using Main L&R outputs but not connecting USB to a computer and instead using a different audio interface's connection to a computer to route the audio back to the computer rather than directly coming from MODX through a USB connection to MODX.

Isolate the noise by using headphones - you hear noise, but audience doesn't

Also, back to your original noisy configuration - if your computer is getting a digital signal from MODX through USB - I imagine this is what your web broadcast listeners hear. The clear digital signal. It's your own personal monitoring that hears the buzz/hum. Connecting earphones to MODX to isolate the analog audio output from the room can ensure your mic doesn't pick up the buzz/hum sound. You'll hear it - but the audience won't. That may be a quick-and-dirty "fix".

BTW: This thread is currently (as of this post) in the "Montage" category and would be better moved to the "MODX" category.