It's been there for a long time, not changing in size or color. Looks like it belongs there, I guess I won't worry, my aro doesn't seem too worried. He's tippin the scales at 18" now, I can't wait to see just how big they grow 'em round here, he's filled out alot, I see the makings of a Monster......mine will get a little spot from rubbing on the glass sometimes.
got a picture?

aDD SOME MELI-FIX
some fishes just have their own unique markings, thats all. if you really know your fish you'll know signs of illness when you see one....