I don't have a reef tank, but I do have a lot of green algae and a yellow tang. All my fish, including my tang, wouldn't eat anything I got except for brine shrimp; and the tang and other vegetarians would eat lettuce that I put in there for them. I bought everything I could find, frozen, flakes, fresh shrimp, squid, etc! They only would eat the shrimp and some would eat the lettuce. Well they were all fatish, but not nourished correctly. Tang was pale and his fins where jagged. I finally did what all good mom's do! I took the fish off their favorite food [brine shrimp] and started preparing a large mixed variety of foods [about 7], added vitimins, and placed amts of this at a time in the tank with a suringe. Well they balked for a few days, and wouldn't eat but then they started eating. Now they have a well balanced diet. Everyone's color improved and the tang's fins are normal now! I started adding their favorite food [shrimp] back to their diet too [but in limited amts]. Now they eat everything!
Also, let me add for comment from all the experienced hobbyists out there....do you believe that a vegetarian fish and kill themselves by eating too much lettuce??? Yes, indeed, it has happened.