All koi root around if they can, at some point. The fatter they are, the less likely that are to look for hard to get food.
All mud has algae in it. Koi eat algae.
But our prepared foods are higher in calories (can you say chocolate?) and so they get accustomed to the nummier food.
Koi, from carp, are actually bottom feeders. They suck on the mud and roots. In a mud pond they wallow around (which helps keep their skin clear of pathogens).
The LIKE it.