A tuxedo/halloween urchin will do a better job than most fish, just make sure any frags are well glued. If you really want a fish, a rainford's goby should do some decent work. Algae blennies are also an option. However, I suggest that you find the root cause of the algae, and then remove that instead of adding an animal. The algae should melt away in time if you do this. Common culprits are excess lighting and high nutrients. I hope that this helps!