Add ACFs to Yoast SEO page analysis.

acf-yoast-add-on.php

function add_custom_content_to_analysis( $content ) {
	global $post;
	$custom = get_post_custom( $post->ID );
	$custom_content = '';
	foreach( $custom as $field )
	{
		$custom_content .= $field[0].' ';
	}
	return $content . ' ' . $custom_content;
}
add_filter( 'wpseo_pre_analysis_post_content', 'add_custom_content_to_analysis' );

// source
// http://wordpress.org/support/topic/how-to-wpseo_pre_analysis_post_content#post-4843463

Awesome!