I realize this post has been around a while, but I found an article that really digs into phosphate and the use of Lanthanum Chloride to treat it. In his description of the effect of phosphate on corals, he states
"The reason for this is that this nutrient has a negative effect on the physiology of zooxanthellate Scleractinia in two ways; on the one hand it encourages the proliferation of brown symbiotic algae in the coral tissue, masking the natural color pigments of the corals and causing the latter to turn brown. And on the other hand it restricts the calcium carbonate uptake necessary to enable the skeleton to grow."
The article is five pages long, but well worth the read. Here is a link to it:
http://www.coralmagazine-us.com/cont...ling-phosphate