I am not sure about the diet of the moorish idol, but I know it is one of the most difficult fishes to keep, and is recomended for expert aquarists. It seems that you know what you are doing for you have had your tank for almost three years but I am not sure if a 55 gallon tank is big eneugh for a moorish idol for most sites and magazines recomend a tank that is over 100 gallons! These fish are very peaceful and can be placed in a reef aquarium but with caution. I have a magazine that has some info on the moorish idol and it states that the moorish idol eat algae and sponges