I think it's meant to be a joke because the character speaking has no mouth. 'Secret' can mean 'hidden'. The speaker is projecting the state of his own lack of lips onto the guy in the white shirt. It's similar in meaning to calling someone bald when you have no hair yourself.

It doesn't make sense to me either it's probably a script writers way to add more nonsense to the script. He seems to be saying something rude in keeping with his character. When I saw the film I thought the character used a lot of nonsense chatter.