The newest release of Mac OS High Sierra is incompatible with our current Alice 2.4.3 release.  Luckily we have a new build that is almost ready for release that seems to fix the issue along with including a lot of other great improvements.  We are still working through several bugs related to this build as it is a major upgrade to our Java version and requirements and JOGL our rendering engine.  If you have this issue please bear with us as we finalize this release.  If you are willing to help us by being a beta tester please contact us and let us know here.