Yellow tang side discoloration

[m3rcfh]

Hi guys
Looking for some input on what the white scuffs/discoloration on my yellow tang could be. I attached some pics below, it's only on one side right at the lateral line and a smaller discoloration half inch below the lateral line that can't be seen clearly on the pictures. This little guy eats a 1x1" square of nori every morning, and Hikari Mega Marine or Marine Algae every night. I try to soak the food in selcon 2 or 3 times per week too.
Is that a scuff from the swimming in the rockscape or can it be HLLE?

Water paramenters:
Nitrates: 20ppm
Amonia/Nitrites: 0ppm
Temp: 79F
pH 8.0
Tank is 1 year and 2 months old.

I have been having issues with cyano and hair algae, so I have purigen and chemipure in the sump right now.

Thanks everyone for any input/advice

 

[Drewe]

So yellow tangs kinda dull out at night. If this is a new thing you're noticing, I'd wait until the morning since they usually regain their color pretty quickly.