He's been eating something yellow. Since he is able to wander all over the house there's no way to know if he's been eating something that he shouldn't...like a yellow crayon. His beak could stand to be trimmed too.
This is why you shouldn't let him on the floor to romp around . Your tort needs to have his beak trimmed . Take him to a vet that can give him a nice filing .I would not let them use clippers. He is a good looking little dude . You should have a look around and see if you can find some crumbs of the yellow stuff on the floor .