Any pesticide product (which includes herbicides intended for killing plants like moss and algae) that is registered with the Environmental Protection Agency will have been tested for environmental impacts. That said, the EPA cannot test every conceivable exposure for various organisms, so they focus on a handful of species. The impacts evaluated also require that the pesticide be used as directed on the label; they cannot test for every possible method of misuse (like over-applying a pesticide or incorrectly mixing a concentration if using the product requires dilution). Since the label is the law (meaning that it's illegal for a pesticide to be used in a manner which violates the label directions), they evaluate a product based on its label instructions to determine what concentration of ingredients its use would expose people or the environment to.

According to that brand's website, the active ingredient in the Wet & Forget Outdoor product is the chemical alkyl dimethyl benzyl ammonium chloride. The National Pesticide Information Center has a fact sheet about this group of chemicals (called quaternary ammonium compounds) that might be useful; it includes both known human and environmental risks. Pesticides also usually include inert ingredients ("inert" does not actually imply they are harmless) that companies are not required to itemize/disclose on the label, but they are still selected by the manufacturer from a list of options that the EPA has evaluated.

Miri