I have seen this before in fish caught in upstate N.Y ponds and lakes. I checked with the DEC about it. I don't now what the scientific name is for it but they called the problem "white grub". These along with the other parasite they called ironicly "black dot" are part of a cycle that starts with waterfowl droppings. The small fish eat the parasites from the droppings, larger fish eat them and so on back to the birds again. While not astheticly pleasing ( that's an understatement) the DEC said that neither species is harmful to humans so long as the fish is thoroughly
cooked (sushi is not recommended here).
I hope this helps out.
Good luck.
Norm