Chop the crawfish tails into rice-size bits in a food processor. Add the egg yolk and process to blend.
In a skillet, heat the olive oil and sauté the celery, parsley, bell pepper, and green onions until tender. Add the crawfish-egg mixture and warm through.
Add the crawfish stock and bring to a simmer. Add bread cubes a little at a time and allow to absorb the liquid in the pan. The mixture should be damp but not so wet that it doesn't hold together; you may not need all the bread crumbs.
Mix the flour, salt, and Creole seasoning with a fork in a wide bowl.
With a round soup spoon, make balls about an inch and a half in diameter with the crawfish mixture. Roll them in the seasoned flour and shake off the excess. Fry them in vegetable oil at 375 degrees until golden brown.