How are you starting X?
With startX or GDM?
You may need to be root to start either if you have changed anything.
What does your Slack use, Xfree or Xorg?
You can probably run
to reconfigure X
Are you allowing user to startX. Slack comes pretty secure out of the box. You may have to be root to start it until you change permissions or config sudo, or add a user to the wheel group ect.