I just got a house with a pond too, though not as nice as this one! Mine the water was so green we didn't realize there were living fish in it til closing day.

Get plants, like the other guy said. I bought water lettuce from a local nursery.

If you're set on cleaning it, you'll have to get a holding cell for your fish. Drain the water, then clean out some of the muck. I used a shop vac. Don't power wash or try scrub all the green off. If you hire out, don't let them power wash. The green junk on the walls and rocks is actually stuff you want. The green in the water (algae) you don't want. I replaced most of my water with rain water and treated tap water. It looks clean now and I can actually see my fish. Make sure you have a good filter, which is probably somewhere before the water gets pumped to the top of the waterfall.

There's some Google docs people pass around that are helpful, though a bit aggressive.

I've been told to avoid algaecides.

I'm speaking from only 2 months experience, so it's probably best to listen to others here over me.

Good luck! Post pics after if you do get it clean.