From the Super Mario Wiki
A Kuromame ( クロマメ, lit. Black Bean) an obstacle found solely in Super Mario 64. They are tiny black spheres that hover in place a tiny distance off the ground. They usually line the edges of pathways. A Kuromame is motionless and harmless until Mario passes by. Then, it expands to about twice its size and shoots a tiny fireball that chases Mario around. If hit by the flame, Mario will run around incredibly uncontrollably while steadily losing three life pieces.
There are two Kuromames located in the downtown area of Wet-Dry World. Due to a mistake in the level design, however, they cannot spit fire. Even at its shallowest point, the water level in the area is high enough that the two Kuromames are submerged, rendering them harmless.