Go to Wal-Mart & get a quart bottle of 3% hydrogen peroxide for only $0.88. Any drug store would also have it.
Are their fish in there ? If so then do not add more than 2 tbsp / 10gal. Make sure you add it all over and mix it in (use a paddle or something)Hopefully you have a water fall or fountain on it. If so, makes sure it's running full blast when adding the peroxide. Allow to dwell & mix for 15 min (20min max) then perform at least a 50% water change.
Best method if no fish then pour in more peroxide. I emptied a full quart into my pond (~80 -100 gal) and the algae will fizz, eventually turn white/grey die (takes a day or so). Wait about a week then use a rake to remove it.