The spotted mandarins are much easier to get feeding on frozen foods. I just took a green mandarin back to the store because he was getting thinner and thinner. And I have 160lbs of live rock. The refugium is a must, in my opinion. These guys eat pods
I think my fish was pretty skinny when I got him, so I'm planning to try again. I'm in the process of adding a 12 gallon fuge.
I hate to suggest this but, if you can stand overfeeding the tank just for a couple days with frozen mysis. Let some of it land on the sand near where he is sitting, he may pick at it. I had luck getting him to accept frozen mysis using that method (but it's not great for your parameters in the short term).