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.
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