First off, here is the Oxford dictionary on multigerm:
"Designating or relating to varieties of sugar beet in which each seed ball contains several fruits and so gives rise to several seedlings."
https://www.lexico.com/en/definition/multigermThus, I think seed ball is fine (as long as it's multigerm, of course).
Second, the "Dictionary of Plant Breeding" by Rolf H.J. Schlegel apparently includes "bigerm" (as in having two seeds). It is also mentioned there that sugar beet typically has one ovule--in contrast to a multigerm (aggregate) fruit.
Third, "Root and Tuber Crops" by J.E. Bradshaw contains this sentence about processing sugar beet:
"In fact, the complete removal of bigerm seeds was difficult when using the gravity separators widely used during seed processing."
A few lines above that sentence, it says that since "hand thinning was very expensive, mechanically processing
multigerm glomerules into single seeds was used."
If I understand the following correctly, monogerm doesn't require thinning, so it could be preferable:
https://hosstools.com/monogram-vs-multigerm-beet-seedsHope the above is of some use.
Best wishes