Your description sounds like what is called algae (actually is not algae, but that is what is referred to as).
Buy a BIOCIDE such as Biobor JF which will kill it. At least in our area, Academy as well as the marine stores carries it. You will still have to filter out the existing "algae bodies", but should not get additional growth.
Have several filters on hand-- that stuff can sure clog up filters.
In the future, be sure to store with the fuel tank full. That minimizes condensation which reduces the changes of algae growth.