Green wifi light on the plug indicates that it has joined in a workable wifi network. Try to close and repoen the kasa app, or sign out and sign in your kasa account again. If the plug still won't show up n the devices list within KASA, check if your samrphoen is connected to the same 2.4G wifi as the plug then check if you have enable the AP isolationon the router.

If needed, perform a factory reset and set up again from scratch. Go to settings on the phone, check if you can pick up and connect to the plug's default SSID before folowing the configuration stpes http://www.tp-link.com/en/faq-946.html