Tales from the Pond
If it looks like a pond monster and smells like a pond monster, it must be...algae. That thick green goop forming a skin on that pond you use as a scenic location for romantic camping trips may be more dangerous than you think. And this has NOTHING to do with bad horror flicks. Sometimes green goop is toxic goop. How do you find out? By conducting good ol' water quality tests—though if you're not a trained scientist or at least being supervised by one, you might stand clear and read up on your local trophic state index instead.
As an example of how algae can terrorize a body of water, look at the toxic ponds and lakes of the Upper Midwestern United States. Overrun with algae, bodies of water like Wisconsin's Tainter Lake, have been known to kill dogs and induce skin and respiratory problems in humans.
Still not scared? Well, just wait until all that algae congeals into a real pond monster, equipped with gnashing teeth and burning red eyes. Or wait until something even scarier happens: all the local flora and fauna are put at risk, gradually dying one by one until not even the faerie folk can help them. Now that's scary. Treat the water right or you'll soon get bombarded with real life nightmares like those.