From the Super Mario Wiki
Goop Generators are puddles of Goop that cannot be washed away in Super Mario Sunshine. They are always located in the middle of a large quantity of Goop. If Mario attempts to wash away the Goop, the Generator will create more Goop. If Mario sprays the Goop Generator with F.L.U.D.D., a Polluted Piranha will come out and attack. Mario must then defeat the Polluted Piranha to make the Goop Generator and its surrounding Goop disappear, after which the object stuck under it will rise up again. In Episode 1 of Bianco Hills, a Goop Generator launches out goop-covered Piranhabons every few seconds until Mario sprays it with F.L.U.D.D. and the Polluted Piranha appears. If Mario doesn't fight the Polluted Piranha, it will retreat back into the Goop Generator, which will then begin launching Piranhabons again. This is the only Goop Generator in the game that does this. Goop Generators can also spawn Swoopin' Stus. A smaller one in the first episode of Bianco Hills spawns an endless supply of Strollin' Stus, and if sprayed enough, pops like a bubble. This smaller one also does not appear within more goop, but stands alone in the grass in the middle of the village.