Question
how do I get rid of green algea on top of water

Answer
That is a short question with a long answer!  The algae is floating?  Is it fluorescent green or dark green?  If fluorescent, it may be blue-green algae which is really a cyanobacteria and not good.  Adding aeration to the pond usually breaks that up.  As far as regular algae, read over my pond algae page for some ideas.  You don't say if this is a small pond or a big one because that can make a difference in how it's treated.

http://www.fishpondinfo.com/plants/algae2.htm

Good luck!

Robyn Rhudy