Take the coral out and spot treat it with 3% hydrogen peroxide which is the standard stuff you can buy at grocery stores or a pharmacy. If you can keep the coral wet while exposing the algae area to the air you'll be much more successful as you can leave the h202 on longer. So use a large tub with 1/4" tank water in the bottom and submerge all of your coral except the algae spot. Then take a syringe and apply a few drops of h202 once a minute for about ten minutes -- again, longer is better.
For a lot of my zoa frags I've simply had to drop them in a specimen cup filled with h202 and leave them in there for five minutes. You then just pop them back in the tank and after a couple days the algae turns white and disintegrates. On some of them the algae grows back requiring repeat treatments but others have been algae free for months now. My zoas always get irritated by this but the alternative is that the algae grows over them and chokes them out.
Another trick if you just need to treat a single spot is to scrub it to remove any that's visable, kill it with h202 and then put superglue on the spot to seal it so that it can't grow back there.
Good luck!
