Aqui dejo la respuesta para todo aquel que tenga este problema...
ralph ddoraNov 13, 2017 11:09 AM in response to bezanilla
Mac OS X
After updating to macOS High Sierra my cursor was surrounded
by a large rectangle that followed the cursor except when the
rectangle was up against the side of the screen
I was able to remove the annoying rectangle by choosing in succession:
Apple icon > System Preferences > Accessibility > Zoom
> right of Zoom Style: click "Options" > UNcheck "Show preview rectangle"
and the rectangle immediately disappeared.
<Publicación editada por el Moderador>