iNaturalist is primarily for wild organisms, but it doesn't matter if they're macroscopic or not.

There are some iNaturalist projects specializing in microbes (more than just these two). A project isn't necessary to submit observations, but it can help bring it to the attention of others able to verify your identification.

https://www.inaturalist.org/projects/microscopic-microbes

https://www.inaturalist.org/projects/cyanoscope