Flourish Excell might get rid of it in the short(ish) term, but the only way to get rid of it permanently is to maintain better (cleaner) water. It will only become established in water with relatively high levels of disolved organic compounds (Nitrates). I'd physically remove as much as possible, hoover the tank etc., and make sure your lighting is able to support good plant growth, then do some big water changes over the next few weeks, before bothering with Flourish Excel.