the substance from blue-green algae, seen in the native guam chamarro population with ALS, is beta-Methylamino-L-alanine (BMAA), an excitotoxic neurotransmitter.
i believe they found it in extremely high concentrations in their brains while doing tissue analyses. they ingested it, not from eating fish, but from eating fruit bats who ate the seeds of a tree that grow on guam, the entire tree contains blue-green algae, i think it helps them obtain phosphorous if i remember right. wasnt from an ocean-dwelling blue-green algae, but it wouldnt shock me to find out that ocean-dwelling blue-greens had similar excitoneurotoxic compounds as well.