Does the UFER Ground require to be visible?
Wrong citation, right answer

Your inspector is right, even though their NEC citation isn't.  They're after NEC 250.68(A):


  (A) Accessibility. All mechanical elements used to terminate a
  grounding electrode conductor or bonding jumper to a
  grounding electrode shall be accessible.
  
  Exception No. 1: An encased or buried connection to a concrete-encased,
  driven, or buried grounding electrode shall not be required to be accessible.
  
  Exception No. 2: Exothermic or irreversible compression connections
  used at terminations, together with the mechanical means used to attach such terminations to fireproofed structural metal whether or not
  the mechanical means is reversible, shall not be required to be accessible.


So, you'll need to provide that access hatch for the "stub-up" connection from the rebar to the grounding electrode conductor, or provide some other way to access it.  If the connection is irreversable, using exothermic welding or a listed compression connector, instead of being a mechanical clamp, though, then you can cover it over -- this might be worth discussing with your electrician.