MSNBC has an article in which it states it “steals the genes” of algae to make their own chlorophyll. Instead, what seems more likely is that Elysia chlorotica ingests algae and transports the chloroplasts of the algae into its own cells. Only some of these chloroplasts can survive symbiotically within the cells of this particular gastropod, the rest die. What has probably occurred is, because chloroplasts were within cells and then died, was that DNA from the chloroplast became integrated into the chromosomes of the slug including the gametes. These newly integrated genes are passed on to future generations (kind of like an ERV).
Even though they can produce chlorophyll, they cannot carry out photosynthesis until they have ingested enough chloroplasts. How, exactly, they go about producing chlorophyll is a bit curious since it is a very complex molecule with many steps in its synthesis and, in the process, produces a highly toxic precursor molecule. Plants, in fact, have a regulatory mechanism (sometimes, more than one) which tightly controls how much of this precursor molecule is present. It would be rather interesting to see precisely what genes are present in the slugs.
But this becomes more interesting when you look at this in a broader picture. While it may be rare, it is certainly possible for genes to be transferred from one species to another. It happens all the time with retroviruses with multiple host species. While the slugs are certainly capable of producing chlorophyll, is there any advantage to the organism producing chlorophyll if it does not contain the rest of photosystem I or II? Probably not. The adaptationist would ask, “well, why have those genes at all?” and the answer is quite simple. If it does not affect reproduction or survival of the organism, it will not disappear very easily. If the organism evolves (or steals) a mechanism of utilizing the electrons harnessed by chlorophyll, this would be another example of “exaptation” or “preadaptation” as seen in Lenski’s research with bacteria and pretty much every single complex trait.