That looks like anchor worm to me...
Anchorworm (Lernaea elegans)
Description:
The crustacean Lernaea is often called "anchorworm" by aquarists as it anchors deeply in the fish skin with its branched suction organ and has an elongated body without visible limbs. At the back end, there are two sac-like outgrowths where eggs develop.
It takes the eggs between several days and and two weeks to attain maturity. Then they fall off and the larvae hatch. The mother crustacean dies and is repelled from the fish tissue after the eggs have fallen off. The laravae are also parasites and go to the gills of the fish to suck blood. As larvae, they attain sexual maturity there. After mating, the female larvae leave the fish and swim around as planktonic organisms for a short time. Then they find a host and bore their way into its skin.
Treatment:
1. Dimilin Powder
The only known method of killing this parasite, without killing the fish is DIMILIN POWDER which can be used safely at any water temperature and has an action of sterilizing the adult and larval stages of this parasite which insures that all eggs produced, after the application of Dimilin, will not hatch.
Method: Dimilin Powder at the rate of 1 gram per ton of pond water. Measure out the quantity required and mix in a plastic bucket with pond water ensuring that the powder is dissolved then add to the pond in the previous manner. A second dosage may be needed to ensure that the life cycle of the anchor worm has been halted. After this second application the dead adults, which will still be hanging from the fish, can be removed using tweezers but making sure that the hooks, as well as the tail of the anchor worm are removed and then apply a proprietary topical dressing to prevent a secondary infection.
Note
Dimilin also goes under the name diflubenzuron.
2. Potassium Permanganate
There is another way of removing anchor worm but more care has to be taken when removing all parts of the anchor worm which is to mix a strong solution of potassium permanganate crystals of 1 gram into 25 mls of hot water. Mix well until dissolved and then dip the tweezers into this solution prior to the removal of the anchor worm, once the solution touches the body, the anchor worm releases its grip immediately and it can then be lifted clear of the fish and the water. Wipe the end of the tweezers on a clean tissue to remove all traces before attempting to remove another anchor worm.
3. Sera Cyprinopur
Follow the instructions accordingly. Use Sera Baktopur to treat the wounds of the fish after the anchor worms have been pulled out. When pulling anchor worms out of the fish, firmly grasp the tweezers near its base where it is burying to the skin and quickly pull it out.
4. Coumaphos
Coumaphos is an extremely dangerous substance so this may be best administered by mixing one gram on a twenty liter bucket and grabbing at least a liter which makes up for a 5% solution for every 150 gallons of water.
This particular treatment may be best avoided however and try safer options such as dimilin instead.
5. Jungle Anchors Away
Change 25% of the pond water before use.
Use one teaspoon (5 grams) to 40 US gallons. Maintain strong aeration during treatment. Clean measuring device before every use.
Content treats 4,520 US gallons. If needed, safely treat up to three times. Wait six days between treatments. Change 25% of the water before each treatment. Remove activated carbon during use.
Data retrieved from
http://www.drsfostersmith.com/.
6. Trichlorfon
Follow instructions carefully. Trichlorfon is available as Masoten, Metriphonate, Dylox, Neguvon, etc.
