It could be a burn but I wouldn't worry too much about it. It will clear up. 15w or 13w bulbs don't produce that much heat to burn them unless they touched the bulb itself. If you are concerned about it just move the canopy forwards or backwards away from where he is sleeping. I do that all the time with my red eyes if they decide to sleep on the walls at the back. If the mark gets worse put a blob of neosporin (without pain relief) on the area and it should clear up quickly.