Myxobolus cerebralis
The kind of M. cerebralis that comes out of fish is called a "Myxospore". They are sort of round and fat in the middle like a lentil or a convex lens. They are about 10  micrometers across and are made of six cells.  Two are for the harpoon weapons, two are for protecting the Myxospore, and two are for going into a worm after the weapons make a hole. Myxospores from M. cerebralis look a lot like Myxospores from other species that are sometimes in fish skin, muscle, or brains, and so it's hard to tell them apart.