Here are a few identifiers:
Green algae and blue green algae. Blooms can occur in response to. If you see leaves or roots, or distinguishable parts, it’s likely a tiny (and harmless) aquatic plant like duckweed. In reality it’s many tiny bacteria that can’t be seen with the naked eye.
Toxins and/or high cell counts have been. Stringy, silky substances that can be draped over a stick are green algae. They normally look green and sometimes may turn bluish when scums are.