Looks like the rays are biting each other. That along with water issues will cause stress and changes in eating.
This was what I was gonna say when I saw the pic.. but didn't wanna contradict some of you guys who've been doing this for much longer originally said.. either the rays are biting eachother or a tank-mate is. they look abit skinny imo ( idk how long you've had them) may try feeding 'em more along w/ keeping your water quality pristine.