How long have you had them in the tank?
I have only experienced them doing such when they're new in the tank and a little stressed out with the new surroundings.
When they're swimming at the surface of the water are they gasping or just very close to the surface?
Glass surfing is a general term and they do it for a rew reasons, boredom, stress, trying to explore the new area, trying to find a way out, etc.
If they glass surf literally constantly then it could be bad as it means they're looking to get away from their current situation and need to be addressed. For new arrivals its perfectly normal if the aquarium is in good health (cycled, maintained, low nitrates)