We have had fish get caught in it and die.
Filamentous algae is a problem that plagues some ponds, yet never appears in others.
It usually appears in the spring, though in warm climates it can be present year-round. Like other freshwater algae, its spores enter your pond in tap water, rain or via wind transport. Strands of filamentous algae can also be introduced into your pond attached to plant material, such as water lilies and iris. When it has appeared in my ponds, it has been in those sections with the longest exposure to direct sunlight. It is also often found on waterfalls.