안녕 would be the correct form and 안뇽/안농 would be wrong. But lots of Korean girls use 안뇽/안농 instead of 안녕 to make their expression cute. Both expressions could be used spoken or written but you should not use the expression 안뇽/안농 to your boss, teacher or to your king (if there is) of your country. You can only use the expressions to your close friends who are the same age of you or younger.

it means "안녕" hello or bye. it is transformed. Among friends, i sometimes say 안농 or something like that.