According to the product group, dismissing a Web Application Firewall not present alert is not supposed to turn the application tile green. This is done on purpose as to not give the user a false-sense of security. However, the reason the dismiss button exists is so that users who are aware of this and have other measures in place can dismiss the item and turn the application tile green.

There are cases where a WAF may be in use but ASC isn't aware of that. Such as a VM running a network appliance with a built in WAF from a third party.