Send data from controller to hook_form_alter()
I think you should not load the data in your controller but in your hook_form_alter. Redirect the user to a path containing a query parameter in Url, so that the url may look like "http://www.yoursite.com/node/add/movies_movie?guid=123" and change the guid paramter to the value of the movie.

Then in your hook_form_alter:

$request = \Drupal::request();
$guid = $request-&gt;query-&gt;get('guid');

if ($system === "IMDB" &amp;&amp; $guid) {
    $database = new ImdbDatabase;
    $movie = $database-&gt;getDataById($guid);

    // fill in the values of the movie into the form fields
}