mandarin with a lump

[Eyford01]

as the title states i have a spotted mandarin and it has a lump growing on its side that is getting bigger, i've had him for about 6 monthes now and has always eaten frozen food (i feed 2/3 mysis 1/3 brine all soaked in selcon once a day) he is fat active and healthy except for the bump of course and still acts normally foraging around the tank picking at pods and playing peek-a-boo, so any ideas?