The Tasker profile is Event - Phone - Phone Ringing.
I don’t have a Harmony, but I’m guessing the Harmony plugin has a mute function. That would be the associated Task. It should respond relatively quickly.
EDIT: I should be clearer, since you are obviously not well acquainted with Tasker yet.
To do this, the first thing you need is to establish the conditions under which an action should occur (that is, run a task or series of tasks). Those conditions that must be met in order to start a task are collectively called a Profile. So open Tasker, then click the + sign in the lower right of your default Profiles screen to create a new Profile.
Then go to the sequence above. There are options, choose Event as the option. In that screen, choose the Phone group… and in that group, choose the icon called Phone Ringing. You don’t need to add anything to the screen that comes up; tap the upper left corner, the ‘tasker’ icon. That will bring up the Available task list, at the top of which will be New Task +. Tap that. Give it a name (such as “Mute TV”), and tap the check mark.
Now click the + at screen bottom to add an action. Choose PlugIn, choose Harmony if it’s there.
Others will have to give you the specifics from this point as I don’t have a Harmony device to refer to.
FURTHER EDIT: As a bonus, you could have it automatically un-mute when you hang up the phone! After you get this together, play with it a bit and see if you can figure out that process.