How to hook_form_alter() inside paragraph item
For Drupal 8 you can implement hook_field_widget_WIDGET_TYPE_form_alter.

/**  
 * Implements hook_field_widget_WIDGET_TYPE_form_alter().  
 */  
function paragraphs_test_field_widget_entity_reference_paragraphs_form_alter(&amp;$element, &amp;$form_state, $context) {
  if ($element['#paragraph_type'] == 'altered_paragraph') {
    $element['subform']['field_text']['widget'][0]['#title'] = 'Altered title';
  }
} 


see my answer here: How do I alter the behavior of the form fields contained within the paragraph field?