I have a 10" round Caribean blue tang (completely different fish, I know) that likes to swim up to my bed of GSp and rub his side and his bellow on the GSP. I've seen the massive fish swim nearly upside down to get an itch on the top of his head. I've had him two years. Scratching can be a sign of disease, it can also be the fish just has an itch... I don't know why fish rub themselves on my GSP... It's a little concerning.... Maybe they just like to be tickled! ... In your case, really hard to know unless there's visible signs. Some will tell you to take it out, QT it, etc... I feel that's too stressful on the fish and me. . . I simply feed healthy food, keep a clean tank, and try to keep it low stress. Maybe there's something there, maybe not. I have one fish that 7 years old, the rest are all going on 3 years old. When you see visible signs such as not eating, lethargy, not swimming right, things like that. Then it may be time to pull and treat - just my opinion. Others will most likely tell you to pull it and put it in QT now to monitor and possibly do other things to treat for infections, etc.