Looks like a sinus infection to me. Just treated a bird for one, and like yours it just appeared one day. She acted totally fine otherwise, but had this spot on her, just like your bird. It got massive before I got it taken care of. I also put her to herself, especially since she was penned with birds who shared a waterer, and didnt want it spread through my flock. Gave her antibiotics and cleaned it every other day with warm water, and flushed it with VetRX every day I cleaned it. In a little over a week, she was as good as new.
 
And, is it just me or is anyone else dealing with birds having the sneezes? I think the weather over the weekend really took a toll on my birds. It was in the 70's most of last week, and rainy and chilly over the weekend, back up near 80 for most of this week, and now I have a few birds sneezing. And it looks to be more of the chilly rainy weather for this weekend. I started giving everything I have some VetRX, and mixing V&E in the water hoping to snip it before it gets to something worse. And, has anyone else seen a spike in mites on their birds? I have noticed a few on the birds the past few days.
 
~Casey