Rummy Nose odd white mark

One of my five Rummies has a white mark develop over his back. There was recently an outbreak of white spot in my tank which disappeared a few days ago after treatment. None of the other rummies have such a mark. The affected rummy is eating and swimming ok.
Any idea what it could be please ?

It looks like some scales have come off its back but it could be the start of a bacterial or fungal infection.

Monitor it and see if it gets worse over the next few days. If it doesn't then don't worry about it.