Yes, they will eat small aptasia. If it gets too big, they'll ignore it. They can be picky and not eat it sometimes, but they're one of the few creatures that will eat it. By the way, they are social creatures so you should consider getting at least 3 instead of one. It's cruel to keep one by itself. Plus, they'll be more efficient. They will require feeding on their own when the aptasia is gone also, and since they don't eat algae, you should consider shrimp or as mine love, small bits of squid. They're nocturnal, so you won't see them all the time, but with time they may come out during the day. Their antics can be kinda funny, I enjoy mine when I see them.