karasugoi has white stuff apearing on his tail

I recently purchased a 20 inch karasugoi (all black with orange belly)
but i bought it as a mirror carp for only £20, not a bad deal stupid
shop!! lol well he has white stuff slowly working his way up his body
what is it and how can i cure it without moving it to another
container??
also it hasnt eaten off of the surface since i got it so that means no
koi pellets, but he is taking garden worms, sweetcorn, peas, maggots,
snails and other pellets used for fishing can i get him to come up and
feed from the surface??????? i mean hes not the easiest fish to see
being black despite its size.

any help would be excellent
MATT


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matt