If it is an anchor worm there are lots of commercial products to buy...Dimilin is a great one(I've used it with success), also Anchors Away I've heard works. You can pull them off, also a good salt water bath for 15-30 min of 0.6% (2tablespoons per gallon) will help. You don't need to use sea salt that is pretty expensive, go get a box of mortons canning salt (green box) this does not contain YPS which is in most table salts and not so good for fish but will do in a pinch. The problem is there may be eggs and the fish could get more so it would be best to treat the pond, but know how many gallons you have. From what I can see it is a very pretty koi. Good luck hope you get rid of it.