Dreams involving confrontation and violence can often reflect feelings of frustration, anger, or a desire to reclaim control in a situation where you feel wronged or violated. In your dream, Garnet, a character you recognize, may represent an aspect of yourself or a situation in your life that you feel is stealing your identity or authority.
Cutting her head off and the imagery of white goo could symbolize a release of pent-up emotions or a desire to rid yourself of that which feels invasive or controlling. The act of violence in the dream might indicate a need to assert yourself and reclaim your power in a situation that feels unjust.
Consider what aspects of your life may be causing you stress or feelings of being undermined, and think about how you can address these issues constructively. This dream might be urging you to stand up for yourself and establish boundaries in your waking life.