Disclaimer: I make absolutely no claims to know anything at all about aquatic disease, and definitely not saltwater diseases. All opinions offered are based on gut instinct/limited general knowledge and are subject to change. I would suggest getting second opinions before acting based on my advice. I am not responsible in any way for loss resulting from making decisions based on my advice.
Ok, now that that's out of the way, if I saw it in my tank I say it was natural, particularly since nothing has changed in a week. If he's acting fine, I wouldn't worry.
Brian