STEREO/MONO link - LS9

Do I need to go to the Master Layer to change my MONO output level?

Yes, you do.
The MONO master fader is on the Master layer and the Custom Fader layer when you assign the fader to it.

By the way, there is a preference for linking the MONO and the STEREO master faders.
If those levels are always controlled together, you can turn the preference on. By using digital attenuator in the EQ of the MONO or the STEREO master channel, you can change balance between those channels anytime.

(LS9 Owner's Manual, p.195)
"This specifies whether STEREO channel and MONO channel on/off and fader operations will be linked.
If you turn this button on, the STEREO channel on/off setting will be copied to the MONO channel, and subsequently on/off operations will be linked. Faders will be linked while preserving the difference in level."