Well I'll start by saying these critters are not the best choice for beginner hobbyists. Some get very dig, to the point that most aquariums will not support them. They also can run out of food, after the algae is gone they starve. Anyone that purchases a Sea Hare should have a plan to find it a new home when the time comes & they can release INK if frightened so they should only be kept in peaceful tanks.
That said, these guys are cool. Their diet consists of algae... red, green, brown... doesn't matter they'll eat it. Golfish's post
https://www.saltwaterfish.com/vb/showthread.php?t=163708 is what inspired me to try one. My 55g has a hair algae problem due to my heavy feeding so this guy (about 2") may be a great solution. This is until he grows up or runs out of food... at which point he already has a new home waiting.