Macro-Algae and agressive snowflake eel

I've got a big problem. I have 55 gallon tank with a bunch of live
rock, matel haylides, Red Sea skimmer (the big one), powerheads,etc.
The water parameters are fine. I've had this tank for 5 years with no
problems. I've had a Hippo and Powder Tang for about 3 years and they
took care of the algae. All was fine until -stupid me- introduced a
baby snowflake. He was fine when he was little, but about a month ago,
he killed both Tangs and a lawnmower blenny. He's about 1 1/2 feet long
and I feed him 3 times a week, so he isn't starving. Since Percy
killed the fish, my tank looks like a kelp forrest unless I pull it all
out. Are there any algae eating/bad tasting fish out there? I would
really appreciate any ideas.