Its searching for food. They are constantly on the hunt for food with the high metabolism they have. They are just treating the side of the tank like the bottom. It's just another surface to trap food against. No worries everything's fine.
ive got 2 motoro rays. and the larger of the 2 does theat sometimes. im not sure what it means.my theory is its doing it either due to too small an aquarium. or its bored. rays (and sharks) are considered very very intelligent creatures and they are probably pinning to escape into the vast rivers of the Amazon where they belong. . . just my theory.