Off the top of my head, it appears to be a fungal issue, but we need so much more information. Tell us about your pond, how big, how many fish, how old is the pond, how is it filtered, do any other fish show symptoms, etc. The more info you can share the more helpful the responses will be.
Welcome to the GPF by the way - hopefully we can help sort this one out and you'll stick around to share and enjoy talking ponds with us!