This would be a simple matter of regulating the maximum quantity of oil that can be stored. That is quite a different problem from what we face in the gulf. And there is no need for the Exxon Valdez when the oil source is 80 miles offshore.
Would you drink motor oil? Don't try to spin this as if there is no difference between crude oil, and food. That is a ludicrous position to assume.
The algae plume cannot exist without the proper nutrients. The majority of the stuff would die and sink to the bottom of the ocean; just as happens already in the normal CO2 sequestration process naturally provided by algae.
The potential problem of releasing bioengineered strains of algae into the wild, is another concern. But I would prefer that discussion be redirected to a dedicated thread, as that is a huge topic generally for all of biology. One immediate thought that comes to mind is that, if algae are famous for doing anything, it is mutating. Given the countless strains of algae found around the world. And considering the existing rate of mutation for natural algae, it seems that we would be hard-pressed, by many orders of magnitude, to pose a greater threat than already exists in nature, to produce a dangerous strain of algae. We could also design strains to be safe. Nature has no such motivation. In fact, it is my understanding that algae essentially have wars when strains are competing the wild. In effect, each strain mutates until one produces something toxic to the other.