The URL for the landing page shows as "https" even though you do not have SSL set up on your Marketo instance, causing the browser to display a "Not Secure" warning.
This can happen if the primary domain and DNS are SSL secure, but Marketo is not. For instance, if your primary domain is "https://www.mycompany.com" (SSL secure) then the DNS, which is also SSL secure, will push down the "https" transfer protocol down to all the CNAMEs on that DNS. This will force the Marketo landing page using the CNAME to use "https" in the URL, even though it is not secure.
There are two ways to resolve this:
Work with your IT department to see if there is a non-SSL option for your DNS
Purchase SSL for your Marketo instance so that both your primary domain and your Marketo pages are SSL secure.
If you would like to add SSL to your Marketo instance, please contact your Account Manager to see about adding that to your subscription.