Callitriche stagnalis

Callitriche stagnalis is a perennial aquatic vascular plant species. Also known as pond water-starwort, C. stagnalis, may thrive in a variety of aquatic and subaquatic habitats, specially those exhibiting slowly moving to non-moving water. Although C. stagnalis does not pose a threat to humans, its reproductive rate may pose a threat to native vegetation in areas where it has been introduced, as the resulting dense vegetative mats frequently out-compete native species....read more on Wikipedia.