Disable text box with preedit text when using an input source and searching in Finder
The box is annoying to be sure.  If you are in the Finder and have clicked upon an editable field, or are in a position to type keys that will result int he Quick Search as you have termed it, this box may appear.

If you are using an input keyboard that supports it, this box allows you to type in the "special" characters required by other languages, and use them for your search.

If you leave some characters in this box, and switch to the US input keyboard, this box will be amazingly persistent, yet will be uneditable while in the US Keyboard.

If you want the box to go away, select the appropriate non-US keyboard, delete the contents of the box, and when you switch back to the US keyboard, it will not appear.