Brown Algae Reappearing On My Filter, What Is This?

Three or four weeks ago I removed my filter because the water flow stopped to a small bit going down the side of the filter (normally the water comes out fast and is propelled farther, and just falls into the water) I took it apart and inside and outside of it there was slippery, brown algae(?) all over it, and even when I took the 3 parts of the intake part out and found the little propeller, that was covered too. I cleaned that off and it did the trick. Now it's happened again and I can see a lot of the brown stuff, I'll clean it again but is there any way to prevent this in the future?