I had this problem for a while now. The brown slime as I call it...

This nickname is universal, really. Yes, brown slime algae.

dead algae or bacteria? Im lost here. The water is clear, the fish are
happy. I backflush the caniter filter once a week. I have uvb. It is
thinner than it was last week. So is this a bio filter getting better?

Well, I don't think anything is wrong with your pond. It's
a sign of life, a good thing.

I think I read somewhere that certain types of algae -- usually
the "green water" variety -- thrive when nitrates are high, but
other nutrients are low. Plant competition may help reduce your
algae, but only if your plants have the appropriate growth nutrients
for the /plants/, otherwise the algae may dominate.

Perhaps someone else can chip in on the missing nutrient. Was it
potassium, perhaps? A little potash, perhaps? Take care, and
look into this carefully before adding any nutrient to your
currently clear pond with happy fish.

You might try adding more of your favorite type of water plant,
and using your pond brush and/or vacuum on the brown slime.

Or learn to accept the brown slime.

C//