From the Super Mario Wiki
Their bodies are filled with
Goop.
are transparent creatures that only appear in
Super Mario Sunshine. Swoopin' Stu can be found wherever there is toxic
Goop. These transparent creatures resemble octopi. Swoopin' Stu come in a variety of colors, and once they spot
Mario, they'll turn white for a second before pouncing at him. They make a disturbing "bwooww-wop" noise and can get in
Mario's way during battles with
Piranha Plants in the Generator. 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' Stu (unless jumped on while they are in the air). Stomping on them also causes them to occasionally drop water bottles (power-ups that will restore half of F.L.U.D.D.'s tank). Eventually, Swoopin' Stu will pop even without provocation, but will not leave goop. For the most part, unless they see Mario, Swoopin' Stu' will generally wander in an outward direction without any real, apparent purpose and won't create a larger 'goop area' unless they fall off an edge and pop on impact somewhere.
Swoopin' Stus also come in a variety of different colors and sizes, though usually aren't any bigger than Mario. Size and color seems to hold no real difference in their behaviors, location, or otherwise.
