The 'Option' key on macOS is mapped to "Alt" on Linux/.
Alt+left mouse click has a particular meaning on Linux, the only way to intercept it and forward to the remote session is to use an active keyboard and mouse grabbing.
Possible alternative solutions are:
a) Work in fullscreen mode (active grabbing is enabled by default):
a1) Run the NoMachine session in fullscreen mode
b) Disable window grabbing on Linux
Open window settings, select' Behavior' then disable "Special key to move and resize windows". This option could be slightly different named
on various Linux versions, but the concept remains the same.
c) Enable active grabbing in the NoMachine client
c1) Launch the player with the --activegrab option.
For example from command line:
c2) Enable both keyboard and mouse grabbing from the Input panel in the NoMachine menu in the session (ctrl+alt+0 to open it or click on the page peel in the top right of the window):