I'll second the abalone suggestion, they're very very cool. Almost as good at nuisance algae control as a seahare, but seem to be less prone to starving themselves out (seahares eventually eat all the hair algae, then starve, unless they adapt to other food, but more don't than do).
There are some smaller abalones that would do well in a tank this size. I know Elite here in Calgary usually has the ones that are about 2" in size. Mine that I've got is a solid 8" so maybe a little too big for a tank your size, so watch out for the smaller ones. Mine is green, but these smaller ones at Elite that I've seen were more brownish.
Another wicked algae eater I've got is a green urchin. I have not been able to ID the species of urchin as it does not match any of the ones I've been able to research. It looks just like the temperate species urchin "green urchin" but a smaller scale model thereof. Basically, think a decorator urchin or blue tuxedo urchin, but all green.
Seahares are basically the ultimate algae eaters though. Just be advised that they starve once the tank is clean. Although I did have one that took a liking to the nori I feed my tang everyday, so he did fine for almost two years before meeting his end in a powerhead.
(Which was weird, because he crawled onto powerheads without incident all the time, just eventually one time it ended badly.). But I think it's more the exception than the rule, if they won't eat any offered foods you're better off to lend him out, or sell him, to the next tank that has an algae problem.
Anyhow hope these suggestions help..