Yeah its been said that the egg/yolk sac leaves a birth mark, and thats why some rays get a brown spot on their belly. Some rays don't.. kinda like an innie and an outtie?
Feeding is always taboo.. a good varied diet, 3x daily, is really a good feeding schedule. Stay away from a single food, especially fatty seafoods. The more variety the better, and dont feed your rays until they are so stuffed they keep eating. They should still be slightly hungry after each meal. No hips, bones, or veins showing around the disc, means you have a healthy ray!