Word - texto tabla marcado en gris - microsoft word table text highlighted grey

 To confirm that it is a field, press Alt+F9 to toggle on the display of field codes and see if your text is enclosed within a pair of field delimiters { }.  Use Alt+F9 again to toggle off the display of the field codes

If the text is within a field, and you do not want the text to be in a field, you can unlink the field by pressing CTRL+SHIFT+F9 when you have the selection in the text.  Otherwise, you can turn-off the field shading via File>Options>Advanced>Show document content and set the field shading to Never to turn it off completely or to When selected to show the shading when the selection is in a field.