A rather odd couple I think you'll agree. We've obviously got a stingray above, a southern stingray to be precise, along with a buddy. A shadow or wingman if you will. It is a jack species and if I'm perfectly honest I'm not sure which one.
What is going on in these shots is something I saw reasonably frequently. The jack is using the ray to find food. Below you can see the ray stirring up the sand bottom as it feeds on crustaceans, molluscs and what not within the seagrass meadow.
As it does this it will disturb some things it probably won't eat. The grass is full of weird little green fish; wrasses, filefish and small snapper are all found in vast numbers either sheltered amongst or camouflaged as the grass. As the stingray feeds, these will move away and the jack will (try to) speed in and grab them.
By positioning itself above the ray as it has the jack is in prime position to take advantage; he is close to the source of the disturbance (the ray) and can use it as cover from which to surprise prey. I imagine the ray gets very little out of the situation beyond an irritating movement in its peripheral vision and some company.