Sea Hare

Sorry for posting this message twice. I accidentally captioned my
last post with a mistake in the subject line and thought it might get
caught in sporge onslaught!

Recently I have been battling a hair algae problem.

I put in a sea hare and phosphate remover and within a couple of days
all was good. The sea hare was healthy, but after about a week, he
just seemed to dissappear. About a month later, I put another sea
hare in. He was there for approximately 2 months or more, moving all
over the tank, and then the same thing happened. One day, I didn't
notice thim there and that went onto several days, and it looks like
this one vanished into thin air.

Are these things hardy, do they die with the lack of hair algae to
feed on (He seemed to be feeding off the glass and the rocks).