Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your query about the nail in your ceiling.
I suspect that the vibration of the ceiling fan has loosened the paint over the nail head. I suggest observing the fan assembly when it is turned on. If the assembly seems to be vibrating too much tightening the mounting screws for the fan assembly should be sufficient to secure it. However, if after securing the mounting screws and you still notice excess vibration, you may need to add extra mounting screws to the ceiling plasterboard to secure it to the frame.
It could also be that the paint on top of the nail was not secure and has fallen off. In this case, you just need to apply some touch-up paint to cover it.
Please keep us updated, we look forward to seeing your ceiling repaired.