very good chance that the ray is just acclimating itself. I will admit that my motoro was very pink on the bottom when I had my gravel. Now without gravel she seems ok, all white, minimal pink. Give it some time and if it remains then make a decision with the gravel.
I'm going to definetly keep my eye on it. For the time being, however, the gravel is staying. I have noticed that he will bury himself in the gravel when I approach the tank to quickly or slam the cabinet door shut. I'd hate to imagine the subsequent stress, had there been no gravel available.