My purple urchin doesnt eat the bad algea. It has grown tremendously since I got the rock so I know its healthy. I have the same algea issue and honestly, Since the pepps didnt eat the aiptasia i wasnt going to count on anyhting eating this algea. So i started the water change, got the tank water level low, ans started pullin out clumps of this stuff. I know I didnt get it all, but I sure did get most of it. With time, this stuff will go away.
Get the fish you want to have based on what you want not what you want them to eat. Remember, they may eat it in the wild, but it doesnt mean that they will eat it in your tank.
BTW, GL on saturday afternoon, your down to a 1 game season for all the marbles.