My white crowntail has a grey patch on his mouth pretty much since we got him from the LFS. Didn't really notice it when he was at the fish store though.
Anyways, he's had it for more than a month now and it doesn't seem to affect him at all. Health is great, playful, flares open when I show a little miss nemo toy on the outside of the tank, loves eating (pellets, flakes, mysis shrimps, blood worms, mosquito larvae) and inspecting all over the gravel for any food remains.
Below is a photo that I took of him in his QT maybe a week after I noticed it.
He's back in his filtered, heater and light equipped tank (14L), with about 30% weekly water change using RODI water.
Until now there's no change to the size of the grey thingy and his behaviour's still great.
In the QT I did 100% water changes everyday with aquarium salt dosed in as well as Melafix. Did this for about a week but nothing changed?