If this the stuff put out by Chem-Marin, it was useless when I used it. When the dosage stopped, the parasites were back. Greenex along with garlic extreme took care of the problem permanently (recommended use of both was LFS owner).
However, anytime you are using something like this, it should be used in a Q/T tank. I think Greenex claims to be reef safe, but I would be hesitant on putting it into a reef tank. I had the problem several months ago when I had to move the fish due to a leak. However, the only coral that I had at that time was put into another established tank and the parasites on the fish didn't show up until several days after the move.
I think I remember seeing a thread in one of the forums a week or two ago on the use of Greenex in reef tanks.