Need help achieving HAProxy active/passive configuration for mysql
The answer is to use the check backup option to second backend node in the conf file like so:

listen mysql-cluster
    bind 0.0.0.0:3306
    mode tcp
    option mysql-check user haproxy_check
    balance roundrobin
    server mysql-1 10.51.120.28:3306 check
    server mysql-2 10.51.123.25:3306 check backup