Thoughts on what this may be?

I am starting a few sealed ecosystems.

Gathered from two water sources after 9pm when human activity subsides.

Temperature was in the 80s. In Southern New England USA.
One a pond. One a river.

The pictures are from the pond ecosystem.

I do not know what this is but it may be one of the following.

A plant in which has deceased and the seeds remain.
An invertebrate or insect which has laid eggs.
A Bryozoan colony.

Quality isn't perfect but I just use the s24 Ultra.

What do you think this is??