The old percula that I used to have did the same thing when I got next to my heater. I'm a strong believer that every fish has its own personality. Your clown might just be more on the aggressive side compared to others. Is it hosting the area where you are cleaning? Mine used to host the heater in my tank for some reason. Try feeding when your getting ready to clean the side of the tank he is on. What size tank do you have for it?