In the wild, the Sea Hare can be found in shallow, sandy, shaded areas. In your tank, the Sea Hare will require lots of space to move, as well as plenty of live rock and sandy area to graze. The Sea Hare requires cover from light in your tank.
Sea Hares produce a dark purple dye when startled, it can become toxic to the rest of your tank if not cared to immediately. You should consider having a good chemical filter system in your tank to remove these toxins quickly.
The Sea Hare will eat Caulerpa; if/when that is not available, they will also eat parboiled lettuce and dry kelp.