I’m not very familiar with Chrome, but I can see “Certfificate (Valid)” on your screenshot, so it doesn’t sound like it would be completely wrong. Check if the page uses some http (not https) resource (image, style, script).
This is indeed a mixed content warning. The connection to the page is secure, but the page requests insecure elements such as scripts which means the integrity of the page cannot be trusted as the insecure scripts could modify it.