its not hystrix but its nice all the same.
its not fat but there is room for not eating yet IMO.
i think its probably just settling in . do small water changes. 25% or less .
keep trying with defrosted prawn ( break it up into small pieces and make sure enough gets to the bottom that the ray can get some ). remove any uneaten food.
if your really worried get some earth worms and chop them up small . the ray should eath those.
good luck