Is There Anything Wrong With My Cory?

[MaxPower]

One of my Sterbai corydoras has had a mark on it from when I got it about 3 months ago, I assumed this was just part of its markings, but recently it appears to have gotten bigger. Can anyone tell me what it is and whether it is or will be a problem to him or any of the others (I hope that sentence makes sense). None of the other corys have this and they all appear to be fine with no signs of distress.

The whiteish blemish is what I'm talking about if its not blindingly obvious.

Thanks for your help.

 

[jelloz]

Incredibly, I also have a Sterbai with a mark in almost the same place. It seems to shrink the more I do water changes but it never totally goes away. It also moves around slightly.
Same cicumstances. Other cories and other Sterbai in the tank are affected.

If you like, PM me with the state in which you bought the Sterbai.

Thanks