cursor-in-non-selected-windows is a variable defined in `C source code'.
Its value is

  • Automatically becomes buffer-local when set.

Non-nil means show a cursor in non-selected windows.
If nil, only shows a cursor in the selected window.
If t, displays a cursor related to the usual cursor type
(a solid box becomes hollow, a bar becomes a narrower bar).
You can also specify the cursor type as in the `cursor-type' variable.
Use Custom to set this variable and update the display."

You can customize this variable.