My Clown Loach has a blue spot on his side?

Red Panda

New member
I have 3 clown loaches in my big community aquarium and Ive had them for about 3 years. The largest one has a blue spot on his side about the size of your pinky fingernail, hes had it since I got him but I never really thought to ask what it was. The tank has a good filter, heater and water quality, what could it be?
Btw, none of the other clown loaches have anything similar.
Its like a bright sky blue
 
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