I have heard that black tahitian moon sand is bad for rays many a time. But I picked up 3 captive bred retics pups and the breeder has that sand in his rays tanks.
His rays have been on that for like 5 years and no problems. The paren'ts were all over the glass and had not one red mark, and they are breeding on it so it must not be as bad as they say.
He even had the pups on that sand and they had pink bellys cause they were only a few days old, but no red marks or cuts what so ever.