[Crani]

"The story features ubiquitous creatures called Mushi that often display supernatural powers. Mushi are described as beings in touch with the essence of life, far more basic and pure than normal living things. Due to their ephemeral nature most humans are incapable of perceiving Mushi and are oblivious to their existence, but there are a few who possess the ability to see and interact with Mushi. The Mushi depicted in the anime look very similar to floaters."

Got that from Wikipédia, checked with someother websites, and they say the same thing.