Image data is stored as if it were a post, or a CPT, so you can treat it like one.

$album_id = get_the_id();
$img = new WP_Query(array('p'=>$album_id,'post_type'=>'attachment'));
var_dump($img->posts[0]->post_content);

Or, a little more complicated,…

$album_id = get_the_id();
$img = new WP_Query(array('p'=>$album_id,'post_type'=>'attachment'));
if (!empty($img->posts[0])) {
    var_dump($img->posts[0]->post_content);
}

get_the_ID will return the ID of the current post so that will only work on an attachment page. I assume, since that is what you used, that is the context in which this is meant to work.

Reference

https://codex.wordpress.org/Class_Reference/WP_Query