A Swoopin' Stu is a type of transparent creature and a variant of Stu that appears in the game Super Mario Sunshine. Swoopin' Stu can be found wherever there is toxic Graffiti. They resemble transparent, goopish octopuses. For the most part, unless they see Mario, Swoopin' Stus will generally wander in an outward direction without any real, apparent purpose and won't create a larger 'Goop area' unless they fall off a ledge and pop on impact. Petey Piranha's Goop is also capable of creating Swoopin' Stus.
Once a Swoopin' Stu spots Mario, they will turn white for a second before making a "bwooww-wop" noise and swooping at him. They appear prominently during battles with Polluted Piranha Plants spawned from Goop Generators, swooping at Mario whenever they have the chance and disrupting his own attacks on the Polluted Piranha Plant.
Swoopin' Stus can be destroyed with a blast of water from F.L.U.D.D. or by being Jumped on, though the latter method will leave behind a puddle of Goop that can spawn more Swoopin' Stus. This can be avoided, however, if they are jumped on while they are in the air. Stomping on them also causes them to occasionally drop Water Bottles. Eventually, Swoopin' Stus pop even without provocation, but will not leave Goop.