Sea Hares will survive as long as they have plenty of algae to feed on and yes they're reef safe. Once your problem algae is gone, they will starve and die. Usually it's best to trade him off to your lfs for credit or other hobbyists for trade.
Yep, seahares are great to pass around in a reef club. Once all of the algae is completely gone in your own tank, give him to a friend in need.
As stated, they are reef-safe and live very long if you have continous algae for it.