My sea hare started eating nori, and did fine. You can also feed them algae wafers. Only thing is I found you need to break the wafers into smallish crumbs, and hold them right in front otherwise the sea hare doesn't seem to notice them.
(You may notice my use of the past tense, however. I unfortunately lost my sea hare in a powerhead incident in December.
Somehow got its foot stuck in the strainer. I cut away the strainer and I thought it was going to recover, but he was stressed, and I think the worms just kinda sensed that distress during the night and finished the job. I should have tried isolating him.)