I've had a mandarin for 3 years with 45lbs of live rock. He's been fine, but recently I read that mandarins would take Cyclop-eeze (a frozen decapod). There is none to be had locally, so I tried Red Sea's Sweetwater Zooplankton (mostly cultured Daphnia found in a jar, not frozen). The first time I tried it he ignored the food. I had my powerhead off at the time. The next time I put the powerhead on for water movement and he went after any of the Daphnia that bobbed by. He won't leave the rockwork to get the food, but will excitedly take any of it that comes his way.
I suspect that mandarins prefer food that looks and acts like live prey, so you may have luck with other forms of food in their natural state.