Help identifying spot on CBB.

[OBB9]

I purchased this CBB from a well known local shop that’s known for QT 3 days ago. It’s been eating well and integrated with the rest of the population with ease for the most part. My purple tang chased him quite a bit on day one but has since settled aside from the quick jolt every now and then. I noticed this spot on its tail this afternoon. It’s whiteish and slightly raised. I have a lot of rock in my tank where it could have been pinned against by my tang.. thanks in advance for the feedback!

 

[OBB9]

Edited to add: just witnessed one of my clowns bite him on the tail and drag him back ~6-8”.