Yes, it will neutralize any phosphate that the rock releases.

what went wrong
 

You could add some of your old LR back in you the tank if you use Foz Down as well. You could also use seachem stability as well There are a load of products you can use. I use Dr. Tim's one and only nitrifying bacteria. Worked great. Follow directions on which ever product you use.

what went wrong
 

So after 48hrs of blanketing my tank, phosphates down from 2.5 to .25.
Ammonia, nitrates nitrites remain unchanged at 0
PH 8.1

I was lucky enough to be given another 20lbs of live rock bringing total to 65lbs, and another 4"LS.
Finally I added Sachem PhosGuard and Purgin and a sock of charcoal in a canister filter.
The tank remains very cloudy. Is this right?
Thanks everyone

Newbie

You should get a good phosphate test kit I'm assuming because of the numbers you listed it's an api test kit? Putting a blanket on a tank to kill off algae won't decrease phosphates but if anything increase phosphates , think about it if algae dies where does that po4 go? Back to the water colum so right away I'd be looking at my test kit going well thats a bit odd lol get a fresh phosphate test kit when you have the chance next it will make life alot easier in the long run :)

Also have a read here about phosphates as well check out other articles they have :)

http://reefkeeping.com/issues/2006-09/rhf/