From advanced aquarist.by ROBERT TOONEN, Ph.D.
"Furthermore, depending on the species, feeding these animals in captivity can be more difficult than it sounds. For some sea hares, the dietary requirements depend to a large extent on the age, because like many marine invertebrates, they can show marked size-related shifts in their feeding preferences (Pennings 1990). Thus, even knowing the species of the slug, and the fact that it is herbivorous may not be sufficient to provide a suitable environment for the animal if you were unaware of the shifting requirements of the animal as it grows"
And from Bob Fenner
"Sea Hares are wonderful consumers of hair algae. It may aid you in physically removing algae, but will certainly not solve your problem single-handedly. They are messy eats, and minute particles of Hair Algae WILL get released every time they take a bite. Also, their feces will contain partially digested hair algae, and possibly hair algae spores. A sea hare would be a wonderful warrior in your battle against hair algae, but you need to also eliminate the problem that is causing the algae in the first place."