A sequin milkshake is a beverage that only appears in the episode "Patty Hype."
They are milkshakes poured in cups with green sequin on them and contain a white bottom and lid as well as a white bendy straw with red circular lines.
Role in episode
The Krusty Krab is hardly receiving any business and the place is practically a ghost town with spiders and webs. As a result, SpongeBob thinks up a new product to attract customers again called Pretty Patties. Squidward and Mr. Krabs find this laughably stupid and mock him by creating novelties of their own like Sequin Milkshakes and bow-tie french fries.