Help, ill fish?

[BlackKraken]

Yesterday noticed white strange mark on my black moor, top of head/back and top of left eye
Has had ich previously but this is different
Doesn’t seem to be acting different but partner and I are worried

[Autumnsky]

Hi. Welcome. Sorry your fish is not well.

Can you tell us about.
The tank size
Size of fish and what type of filtration?
How long have you had the tank running
and are you familiar with the nitrification

What fish are in the tank?

Have you added any fish recently, made any recent changes?

[Autumnsky]

Oh, have there been any water problems?

Do you have the water parameters for the tank?

Ammonia, nitrite, nitrate?
PH
KH / GH
Anything else

Doing some water changing seems like a good start.

30% pwc, then another, and another. The first 2 one then another within a couple hours. 3rd around 6-8 hours.
It looks like it could be a bacterial infection. Clean fresh water is the start to helping quite a number of fish issues. Aleays match the water temp an use dechlorinator. Prime conditioner is a really good option.