Hello,
Canvas 4 uses JSON Data from the AJAX Forms to identify the Type of Alert in a Better Way. In Version 3.x and below, even a Server Side Error for the AJAX Form displayed a Green Alert. But now with Canvas 4, the Alerts can be displayed as Red or Green based on their Type.
You are seeing this Message as you have not Transitioned from v3.x to v4 completely. To fix this, simply follow the Documentation > Upgrade V3 to V4 > HTML > Contact Forms Section and Update the js/functions.js File to the Latest Version.
This will definitely work fine. Let us know if we can help you with anything else or if you find any further issues.