add_action('post_grid_media', 'post_grid_media_20200614', 10, 2);

function post_grid_media_20200614($post_id, $args){

    $source_id = $args['source_id'] ;
    $source_args = $args['source_args'] ;

    if($source_id == 'something'){
        ?>
        <div class="">Custom media HTML</div>
        <?php
    }
}