Spongy Patties were sponge-like patties that poisoned the customers and turned them yellow and spotted like SpongeBob SquarePants. They were actually just stale, frozen Krabby Patties with a yellow, spongy color.
They were accidentally invented when Mr. Krabs wanted more room for the SpongeBob-shaped ice cubes as part of the Krusty Sponge restaurant. He thought they looked like SpongeBob and told Squidward, the new fry cook, to fry these instead of the normal ones. They only appeared in the episode, "The Krusty Sponge." When a Krusty Sponge customer ate one, he/she became yellow and had a stomach ache. Eventually, an agent caught Mr. Krabs, and he went to court. Spongy Patties were never sold again after that.
The patties are yellow and spotted, like SpongeBob. They are normal Krabby Patties which have gone "stale" and were poisonous to eat. This is what gave them their color. It is unknown if it's holes or dots on their skin.
Mr. Krabs said that the Spongy Patties were Krabby Patties that were left out for awhile. In the episode "Born Again Krabs," another Krabby Patty was left out but it was old and crusty (the color of it was gray) unlike the Spongy Patty, it is possible that the Spongy patties were more of "poisoned", instead of old and crusty.
Ironically, back in that episode, Mr. Krabs himself was poisoned by eating it.