Jr, you know better.
I've had experience with stingrays before, and IME if the animal is in some sort of poor state, your first sign will be the animal seems stressed - i.e. doesn't eat, shy, generally lathargic. From your description, it seems your ray is happy as a clam. You didn't mention if you had any corals or anemones, but I assume you don't. Are the red marks indicative of scratch or rub marks, say the type an animal might get against LR? Most rays have fairly tough skin, but it is a possibility. Doesn't bring to mind any parasites. Just guessing here...sorry. Good luck, hope it is fine.