Re: @Destroy All Monsters
There's a lot of useful mass on the moon to feed a hungry space egg if it's not a breatharian. Lunar soil has been demonstrated as a basis for growing terrestrial crops, albeit with lots of fertilizer and nutrients. More alien critters could get all sorts of material from the soil.
But that doesn't lead to any increase in total mass - your just changing a mass of food (soil etc) into an equivalent mass of dragon chick. The total mass of the system wouldn't increase as the chick grew by consuming the soil.
(As to growing crops on the moon the extra plant mass would mostly come from the C02 in the atmosphere you would need to surround them with + the water you would need to provide them with. Relatively little would come from the soil. )