highlight-changes-passive-string

highlight-changes-passive-string is a variable defined in `hilit-chg.el'.
Its value is

" -Chg"

• This variable is an alias for `highlight-changes-invisible-string'.
• This variable is obsolete since 23.1;
  use `highlight-changes-invisible-string' instead.

Documentation:
The string used when in Highlight Changes mode and changes are hidden.
This should be set to nil if no indication is desired, or to
a string with a leading space.

You can customize this variable.