That looks like head erosion, AKA early stage HLLE.
Looking at this and your other thread, you have a few issues that are all probably playing a part
You have double the recommended amount of carbon and are tumbling it in a reactor. Heavy carbon use is a widely accepted cause of HLLE.
With zeroed out NO3 and PO4 in a tank as sterile looking as yours, it's going to be a testing error, or you are not feeding enough.
Also, what specifically are you feeding? Poor nutrition can play a role here as well
Kole Tangs are more detrivores, so not being interested in Nori is not a deal breaker. They do need to have browsing opportunities and some vegetable matter in their diet. My Kole enjoys pellets with a mix of vegetable matter, and I leave some algae on my back glass and bottom. He will scrape at these areas throughout the day.
My plan of action would be:
Take the carbon offline completely
Get nutrients up by feeding more
Work on a more varied diet and get some Selcon to add to what is being fed.
Good luck!