How to stream Tidal Masters from IMac to Muso

On your iMac, go to System Preferences.
There, click on "Sound" with the little icon of a speaker
Click on the "Output" tab
From there, a drop down list of all available speakers should show, including your Mu-so
Select your Mu-So and your sound will now eminate from the Mu-So via AirPlay

And voilla, you'll have an allegedly good source ruined through airplay. 

Is there a way to hardware the connection? 

If you plug your iMac optical output (if it has one) directly into the optical input of the Muso it should also playback through the Muso.
To be honest, although Airplay is not the last word in hi fidelity, for playback through a Muso, I suspect you'd be happy enough with the results.

You'd need to run UPnP software on your Mac and access the content this way.

Using NAS is a far more common way of streaming to Naim streamers.