This enemy appears to be a Amoeboid, due to its behavior or a Goomba made entirely of blue-colored gelatin. When failed to be defeated in a single attack, this foe splits into five, all having identical stats in which is considered an insignificant annoying enemy. This foe is strong against jumping attacks, but weak to fire attacks.
In the overworld, Ameboid disguises itself as a blue spot. When closer approached, it reveals itself as an Ameboid.
Ameboid's Stats (Super Mario RPG: Legend of the Seven Stars)