I came across these green balls in the lake during the last week of August 2022, and couldn't figure out what they were. Their color (blue-green) made me worry about cyanobacteria, and in this regard I was right. These are a species of Nostoc bacteria that are a subset of cyanobacteria, but toxic they are not. They are apparently common.
They are photosynthetic, but may also grow within the tissues of plants, in a symbiotic relationship. The individual balls are a gelatinous sheath enclosing filaments of cells that resemble a string of beads.
See also Maine Aquatic Field Guide