i had the same problem with my chinese in my wife's tank (haven't talked about it on here yet, i'll get to that). and it has since "disappeared" in my bluegill tank. not just hiding, because there is algea EVERYWHERE. don't know what happened to him, but he was a bit of a jerk anyway.
i recently read that as they get older they eat less and become more annoying (not really aggressive, just a pain).
we bought two gold algea eaters that, according to what i've read, will continue to eat a lot throughout it's life and is a much more playful fish.