Sure looks like FA (filamentous algae) which is present in all ponds to some degree or another.

Go here to the vegetation forums of PB...

http://forums.pondboss.com/ubbthreads.php?ubb=postlist&Board=14&page=1

and search on "FA" for a lot of reading.

FA can be a horrible problem, but it's most always the conditions of the pond/lake not the fact that it's present. Shallow waters, high nutrients, and ample sunlight reaching the bottom are all conditions that promote the slime.

I wonder if your plants are dying back creating some nutrients for the slime to live off of, some partial water changes may help reduce it's growth.

Welcome to forums!

Fish on!,
Noel