How hot is the tank? If it's warm, even with a HOB and sponge, it may not be enough oxygen
That said, many cories do like to search the surface of the water like that. Mine do it a lot in the floating plants. I've got an old video somewhere of my old aenea doing it and you could hear them doing it every single night.
Just looking for food. Ironically mine were also albino aenea