Up to Mac OS X 10.7, Apple was including the X11 app with the OS. Since OS X 10.8 this is not the case anymore, see https://support.apple.com/en-us/HT201341. If you want to use X11 under OS X 10.8 and newer, you have to download and install XQuartz from https://www.xquartz.org/.
To be compatible with recent X11 libs on the linux side we recoomend to use XQuartz version 2.7.10 or newer.
About the ssh configuration on the Mac
X11 Connection Expiring
Since Mac OS X 10.7 (?) the ssh has a very short timeout value for untrusted X11 forwarding ("ssh -X"). As a result the X11 connection expires 20 minutes after you connected to the remote host. If you suffer from expired X11 connections you should add the line
to the "HOST *" or "HOST lxplus" section of the file /etc/ssh/ssh_config (/etc/ssh_config for OS X up to 10.10.x) or ~/.ssh/config. Alternatively you can use "ssh -Y".
Messages about unsupported 'locale'
When you receive messages like the following
perl: warning: Setting locale failed. perl: warning: Please check that your locale settings: LANGUAGE = (unset), LC_ALL = (unset), LC_CTYPE = "UTF-8", LANG = "en_US.UTF-8"
you might want to comment out the line
" SendEnv LANG LC_*"
from the file "/etc/ssh/ssh_config" (admin privileges required).