...commonly known as "dog vomit" slime mold, among other colorful common names.
At first I was excited! Slime mold! I don't see if very often, and last time, I wasn't quick enough to collect some.
Turns out, it's not particularly exciting under the microscope. It's a colony or a collective organism, or something, and it's made of many tiny individuals.
Then something awesome happened. The slime mold body floated really well, probably because it's a big loose structure of what I'm calling spores. I don't know if this was before or after I put it in water, but the cells all clumped around tiny air bubbles: