Rays that young will likely only accept black worms.. It might take some time in captivity for them to get used to pellets.
My juvie retics are only eatting live blackworms, red wigglers, and chopped earthworms.. I -think- they are eating HBH supersoft pellets mixed w/ their blackworms, but are refusing shrimp, mussels, and other dead foods.
They have tiny mouths, being one of the smallest species of rays.. This makes it hard for them to take larger chunkier foods. They are also VERY juvenile, so starving them into accepting other foods isn't an option until they grow out a little bit..