Universal Control is for and will control "only" audio I/O for the RM16.
The Lavalier mic you have connected to your computer "built in" mic input is using your Mac's built in audio hardware, not the RM16.
2 different audio processing and hardware.
You can create an Aggregate Device that might allow you to use the I/O of both the Lavalier mic and the RM16 within a DAW software, but "not" in Universal Control.
Again, Universal Control is exclusive to the RM16.
A better solution is to use a better mic that you can connect directly to the RM16.