This is Wild Caught from FRP? It's from stress bad conditions from exporter to seller more then likely the ray has hook in it and is stressed out from pain and not eating etc. This is why I keep any wild caught import for few weeks before selling to customers. If get them in ok they will do this within first week or two if there are problems. Nothing you can do add salt incase there is a hook in belly to help with any possible infection. Once they get this there is a chance it will go away but takes long long time. Good luck.