Cecil while alum and gypsum can tie up phosphorus I would never bet on it reducing algae (FA).There are several problems with that approach. One it can also clear the water letting more sun get to the FA causing an algae explosion. Two the phosphorous removed into the floc which settles out is carried down to the FA growing on the bottom. Three you have to know your water chemistry (what limiting factor N,P,K or other is present) and how much of your P is removed (tied up) by the alum. If alum is tried and your guess on any of the points is wrong you can easily be much worse off than before especially in low alkalinity waters.

There are much better options for FA control than alum not to mention the difficulty of use and cost.