my dad told me it's what all goldfish do. since they eat EVERYTHING, when they can't get food they improvise and eat the tasty things growing on the rocks.
unless he starts taking one to you at the front/top of the bowl and begging you to throw it, i think he's fine =)
whoo... i know Comets and all Fancies CAN grow to 28-20", but how fast? uh... slowly? i had a pair of Comets for well over 5 years before they met untimeley demise (cat fangs and overfeeding on my brothers part) and they never grew past 2" and 3" respectively.
i do believe keeping goldfish in smaller tanks stunts their growth a bit, they do grow, but they grow much, much, MUCH slower.