The fish most likely has a fungal disease (Saprolegnia)
This looks like fluffy white cotton wool-like growths on flanks or wherever the skin has been damaged. It can sometimes have a green tinge when algae are also present in the water.
Not to be confused with:
Columnaris disease (cotton mouth) often seen as mouth rot and also around the edges of scales and fins causing the tissue to rot away.
If your Koi looks like the illustration, we recommend you use one of the products listed to help restore your fish to good health.
Take a look at our full Koi Disease Mat.
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