Uronema heteromarinum

This is a ciliate in the order Philasterida, the group of parasites including all known causes of the disease called Uronema or Scuticociliatosis. 

Because of its close genetic relationship with these known parasites, its morphological similarity, and its similar prevalence in the hobby, we label this a suspected parasite: we suspect it contributes to the Uronema observed in the hobby. 

Interestingly, this ciliate has been repeatedly associated with coral diseases in the research literature. 

U. heteromarinum is the most common suspected parasite found in hobbyist samples, occurring in about 8% of the samples we’ve tested.

For more details, see this discussion about the disease.