In the new 1.13 snapshots, the cursor appears in the middle of the screen when getting a notification in macOS. After that you can still play, the cursor just stays in the middle of the screen.
But it shouldn't be there...
(In the short demonstration video clip I'm getting a notification from iTunes when the mouse cursor appears.)
Is this still an issue in the most recent versions (currently that is 1.13.2, or the latest snapshot 18w49a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.