How to render a field from loadtree
I Changed how to build my form,

public function buildForm(array $form, FormStateInterface $form_state) {
  $terms = \Drupal::service('entity_type.manager')
      -&gt;getStorage("taxonomy_term")
      -&gt;loadTree('faq', $parent = 0, $max_depth = 1, $load_entities = TRUE);
  $terms_select = array();
  foreach ($terms as $term){

      $question = $term-&gt;get('field_question_faq')-&gt;getValue();

      $terms_select[] =  $question[0]['value']);


  }


and then everything was fine!!

Thx to Vikram8888