What does "MERMA" mean on the Los Angeles TAC chart?
MERMA is an aviation waypoint


  Waypoints used in aviation are given five-letter names. These names are meant to be pronounceable or have a mnemonic value, so that they may easily be conveyed by voice. 


According to this page back before the FAA decided all waypoints must have 5 character names, this waypoint - being out in the ocean - was called "Mermaid" and after renaming this became simply MERMA.