That's probably not normal. It could be the start of a fungus or columnaris or he may have scraped his head on something.. To treat conservatively, add 1 tsp per gallon predissolved aquarium salt to his water. Make sure to keep it very clean.. Water change procedures are here depending on size: http://www.bettafish.com/showthread.php?t=66595
That general forum also has a lot of stickies on care, in case you might be new to this :)
You can treat for up to 10 days so long as it doesn't get any worse. If it starts to get worse or it's not gone by 10 days I'd try Kanaplex by Seachem.
Good luck! Overall he looks healthy :)