For my $0.02 worth:
This is rare, but I have been keeping a blue female mandarin in my 29 for 5-6 years now. She loves fresh blood worms, happily eats frozen mysis and bloodworms, also chows down on sinking pellets mostly. I went through 4 kinds of pellet food until I found one she really liked.
You must feed them daily. Mandarins are the hummingbird of the ocean (and our tanks) they use so much energy in their quest for food that they must eat small portions daily. I feed her every day.
Not all mandarins like the same food, I just got lucky.
If you can't find a way for your mandarin to eat, please give it to someone with a larger established tank screaming with pods.
GL