Try frozen clams on the half shell (LFS sell them) or pick up a live clam whack it with a hammer busting the shell and use that. IME very few fish can resist the clams
moorish idols need to be acclimated first they do not like stress once settled start Mysis and brine do not give up also try very small bits of clam mix many foods together in a broth they love the smell once they start they eat everything ....you need to try many times keep good water quality they must be fed frequently
I'd also stress the fact that they need a large tank. These fish are swimmers and will not do well in smaller home tanks. Larger tanks are also more capable of supplying some of the foraging foods that will help them maintain a healthy weight.
I appreciate the article on this species eating sponge but that is not the sole food which this fish survives on they like most reef fish live in an environment that makes them opportunistic feeders it is not apriori that they must start on or have sponge to survive they must have high quality water conditions and appropriate husbandry my personal experience is that they can be easily the most aggressive eaters and will be the first to respond to daily feedings reef fish can almost never be over fed ..is is very important to employ skimming and redundant bio,chemical and mechanical filtering to never compromise water quality..use quality food supplements like selcon/and water soluble vitamins and beta gulcan to enhance immune response the worst is sudden temperature shifts that go unnoticed I kept several idols and they loved soaked pellets and would stick there long nose out of the water to feed ....I believe they are very misunderstood by most of the marine aquatic industry.they gained body mass and where extremely healthy they also love different turf algae
I had one for about 6 months then one day stopped eating and the next day it was dead. I got mine to start eating clams and mussels from the shell. Theres also an angel food that has majority sponge in it. It went crazy
For that too.
I would like to comment on the fishes temper, once they become settled and are doing well they are fairly agro and can make poor tank mates for docile fish. I know of one that tried to kill all gobie's on sight. (oc - many bristle tooth tangs do this also)
while you are down at the dock collecting sponge you can strip pods out of rope ends left in the water. just make the OK sign with the rope in the O then strip into a pail. use ropes that are in the water and look like they have been for awhile. be care full on nylon .. those splinters suck.