Brown slime on my 300 gal. preform

I had this problem for a while now. The brown slime as I call it, Its on
all the sides and all the plastic plants and rocks ect. My preform is
above ground and because its not in the ground it gets warmer than a in
ground preform. The temp today was 78 and water temp is 75 if the
outside temp goes to 90 it gets to 84. I took a paper towel and wiped
off the sides of the pond. Its brown color. (stain brown like poop
toliet tisse color) Is this dead algae? Its thin and its on all the
rockes. The fish seem fine. The paper towel was browm stained. Is this
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? I
was cleaning the 3 bio filters course, medium ,fine, but now I just
bachflush once a week and the brown slime is better. Any Ideas here? Is
it better to just leave the bio.mech./uvb alone? Thank you