That appears to be Anchor worm. There are a few ways of treating. 1) a bath of Potassium Permanganate. 2) Dimilin will kill anchorworm. 3) Coppersafe. 4) Cupramine. #3 and #4 are copper based so best used in a separate hospital tank.
Another option if there is only one worm is to physically remove the worm with a pair of tweezers then watch carefully for post infections. There may be more worms in intermediate stages that you cannot see yet so you'll have to keep a sharp eye on the fish for reinfection.
Hope this helps