Sea hare is somewhat like either a nudibranch or a cucumber or errrr... a bit of both. They do indeed eat hair algae with a passion. The down side is they do not live long and release a toxic substance similar to some nudibranches that can crash your entire tank. IMO you should find out why you have an algae problem and fix the problem and not just put a band aid on it as a temporary fix. eg:- excess nutrients,lighting schedule, type of water used etc...etc