Green layer of stuff on top of the water in my salt water tank?

I've had this green layer of..... well I don't really know. It's been there for a couple of months, and goes away with water changes, but returns within a few days. I've tried using different water (distilled and tap) and salt, and trying numerous water changes, but nothings happened. I've got a 20 gallon set up with a yellow tang, percula clown, mandarin dragonet, mushroom polyps, a condi-lactis anemone, and emerald crap, and numerous kinds of algea (all being green in color, so the basic). I seem to be getting the problem after putting PhytoPlex into the water. I have bought a different bottle of it and it still persists. I put in the same amount I have before. I put in about a half a teaspoon. I was putting in more previously (about 2 teaspoons) when I was directed to use only half a teaspoon a few months back. What could be my problem?