Agree with kno4te. While waiting for proper pic and feedback I suggest adding salt in the meantime.Hard to tell. Could be a body fluke, leech, or a piece of skin scraped off and moving in the water. Would suggest betting the fish and getting a close up.
It does and it doesn’t look like anchor worm. The base looks like it but the piece hanging off of it doesn’t. Better pic would tell and treatment would be easy if it’s just the one and that’s pulling it off the body.Id say some parasite.kno4te take a look on the yellow thin strings sticking out. Anchor worms?