Was this working properly before? If you create a new profile and start a new game, can you see the mark and move by clicking on the ground?
If you left click and hold, will the character move in the direction of the mouse cursor?
If you hold Left Ctrl to force attack, do you see that mark on the ground?
Try switching to windowed or fake fullscreen display mode, and/or lower the game resolution, to see if that makes a difference.
Try holding the Cmd button on game startup to get to the configuration menu and enable all the workarounds (at least D:OS EE has a 'precise mouse tracking' workaround that can help with various mouse related issues).
Try doing a clean boot and then test the game. Start/restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock.
Try disabling mouse acceleration; that can help with a different mouse issue, predominantly in D:OS EE (clicking through the UI
Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
With the GOG version, in the (optional) Galaxy client, select Divinity: Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'.
With the App Store release, or GOG without the Galaxy client:
bash-3.2$ cd ~/Library/Application\ Support\ ... up to the Divinity Original Sin 2.app folder
bash-3.2$ codesign -vvv Divinity\ -\ Original\ Sin\ 2.app
This takes a while, and will output some lines starting with '--prepared' or '--validated'. If everything is fine, it will end with:
Divinity - Original Sin 2.app: valid on disk
Divinity - Original Sin 2.app: satisfies its Designated Requirement