That's the following language string.
Code:
'UE_USER_CONFIRMED' => 'Your account is now active. You may now log in!',
You can use the new language overrides functionality shown below to override it using the key UE_USER_CONFIRMED.
www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived
Easiest way to find keys is to edit the language PHP file and search it. I'm looking into a way to help find keys though directly within the override parameters usage.