It's a type of obstacle in Sword of Mana found in several dungeons. They consist of a sphere of two colors, which are immobile guarding a place, only when the character gets close to it, they launches an attack, preventing the character's passage. Despite giving experience points like Stone Seals, they are not considered monsters, since they are not included in the bestiary.
According to Popoi's Note about them, they can be of three types: slash, bash and jab. Each metaball inflicts damage based on its type. A metaball's type can be determined by its attack method. They are all resistant against magic attacks of spirits, only certain physical attacks work.
The Slash type made blade-shaped extensions and attacks by spinning, they are resistant against the knucks, the flail, the staff, the bow, the lance, the mace, but weak against the sword, the sickle and the axe.
The Bash type made ball-shaped extensions at the end and attack with a punch, they are resistant against the sword, the flail, the sickle, the bow, the lance and the axe, but weak against the knucks, the staff and the mace.
The Jab type makes pointed projections to attack by stabbing, they are resistant against the sword, the sickle, the staff, the knucks, the mace and the axe, but weak against the bow, the flail and the lance.