No expert here, but I'll take a stab at it.
The short answer - that's filamentous algae and it's easily treated with an algaecide.
The long answer is you have a lot of nutrients in your pond, and as they say - mother nature finds a way. Same as if you had a garden full of rich black dirt and left it alone, only to find it full of weeds.
For long a term solution, you might consider planting some nice plants in your pond of your choice, to use up some of those nutrients, before mother nature does it for you.
