algea removal

i was changing water because it looked bad and was scrubbing down the sides of the tank when i go to change the filter i saw this this is after two weeks and the filter is supposed to last a month or two inside the tank the wall looks like this

Its a bright green algea and it spreads over my substrate (fine gravel) and looks like some sort of filmy membrain but overnight it shrivils up and turns into a slimy brown clump that floats to the top. It then repeats this cycle every few days. sorry I couldent give any pics of what it looks like.