Chummy Joes are a type of food that is sold at the Chum Bucket. They are a parody of Sloppy Joes, a sandwich comprised of ground meat and sauce on a bun. At the end of "Mermaid Man vs. SpongeBob," Plankton told the fish at the Chum Bucket that he would serve these to them, but they had already left for the Krusty Krab.
He created them, only after he realized to himself, he didn't think he'd make it that far. Like all of Plankton's food, these are not safe to eat, and if ingested, will cause health problems.
In the Brazilian dubbing, Chummy Joes are called "Lixoburgers" ("Junkburgers," literally), since the Chum Bucket is called "Balde de Lixo" ("Junk Bucket") and the food (along with the restaurant) is hazardous to eat.
Ironically, Plankton's customers left before anyone digested the Chummy Joes.
Chummy Joes have not been eaten by anyone yet, but it can be assumed that is can cause food poisoning.