Whiteish patch on flame angel, only visible on certain angles

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I’ve currently got a flame back angel in quarantine and noticed a whiteish patch on its upper left side. The weird thing is that you can only see it on certain angles. If you look straight on at the side, it’s invisible. Fish has been in medicated QT for 10 days and it the patch hasn’t changed.

Any idea what this might be?

The angel doesn’t seem to be disturbed by the patch.

 

[Gumbies R Us]

#fishmedics