Only output function from functions.php if conditional tag is true

Move the is_tax() conditional inside the slug_scripts_masonry function.

function slug_scripts_masonry() {
    if ( is_tax() ) {
        wp_enqueue_script( 'masonry' );
        wp_enqueue_style( 'masonry', get_template_directory_uri().'/css/' );
    }
}

add_action( 'wp_enqueue_scripts', 'slug_scripts_masonry' );

Because WordPress has not yet executed the code which determines the result of is_tax() it will return false on all pages.