It is a light brown box with the modern (old) versions of Mermaid Man and Barnacle Boy on it, suggesting it is either a more recent product or the box art simply changed when the heroes came out of retirement.
There is also a large cereal bowl that resembles Mermaid Man's dark green trunks and belt that is probably a tie-in to this product. SpongeBob also eats this cereal with Breakfast Bites which is likely another tie-in.
Role in episode
At the start of the morning, SpongeBob eats them while he watches the Mermaid Man and Barnacle Boy episode. He mostly likes them because Mermaid Man and Barnacle Boy are on the box.
Bran Flakes is very similar; they're possibly the same thing but with Mermaid Man and Barnacle Boy on the box, or else made by the same company/manufacturer.