Lightbox Caption Showing Filename – How do I change that?

there is one problem with your links on that masonry – the titles are often the same as the file-names
your alt attribute is always the same for that gallery: 7106 Bluff Run

Next : on the element itself – where you set the option to show lightbox images. – there is underneath a dropdown with an option what to show as bottom-bar text:

But if you have forgotton to show this on multiple masonry galleries – then you can place a little snippet instead in your child-theme functions.php:

function masonry_title_fix(){
?>
<script>
(function($){
    $(window).load(function() {
        $('.av-masonry .lightbox-added').each(function(){
            var lin = $(this).attr('alt');  /* === or choose title here === */
            $(this).attr('title',lin);
        });
    });
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'masonry_title_fix');

because you have the masonry perfect grid – you have no imgs inside but you can change it for the anchor tag.