If you don't see anything and other fish in tank are clean, do nothing. I had a banana wrasse that would scratch every day for over 6 months. (traded it in due to aggression) At a LFS they had 4 big ones in and all of them scratched. They may just do it....
After reading some of the threads in this section I am surprised that so many threads go unanswered. I am never in this section because my fish aren't sick and I can't use the search function which makes this section useless.
I will share this though, often times over reaction to what you think is a problem ends up being more deadly than leaving the fish alone. Good water and good food come first.
I have noticed that ich comes and goes for many fish and eventually goes away on its own. A new purple tang was covered in ich for 2 weeks and then finally went away. Same happens with wrasses, angels and triggers that I've had. I do nothing and it will go away with the help of 2 cleaner shrimps on duty.
Marine velvet though is another story though....acraflavine is the only thing I've used that worked.
