ID'd by Paul Hamilton of the Canadian Museum of Nature, who commented:
"This is a green alga (Chlorophyta) in the genus Botrydium, probably
B. granulatum (L.) Greville. I would need to see the cell structure.
This taxon will form large mats of these globules along dried shoreline.
If you pick one up you would see a fine holdfast structure on the bottom
of each globule. Most often observed during periods of drought."
In fact, this algae did occur after a dry summer during which water levels
in the beaver pond had dropped significantly.