If it doesn't look like a growth then its probably a scratch. My blue tang had the same thing when i first got it. They try to make a burro/home in the rocks by rubbing themselves against it. The white mark went away about a week after he stopped making his home, just make sure the water is clean so it doesn't get infected.
Edit: Yea its normal nothing out of the ordinary like i said they try making homes in the rock just let him be and make sure the water is clean so it can heal when he is done.