Maayong adlawng natawhan is the Cebuano translation for the words happy Birthday. It is a shortcut to maayo nga adlaw nga natawhan, which the natives do not use much. To mean the same, you can also use Malipayong adlawng natawhan or Malipayon nga adlaw nga natawhan.
ah. just say happy birthday and they'll understand it just fine. i'm cebuana and believe me that is how we greet a birthday girl/boy. some say it in cebuano to be unique because most of us say happy birthday. but its up to you, you can say "maayong adlawng natawhan" for a change too and we won't really mind.