Traci is spot on (as always!)
My Mustashio had the same thing and I worried about it all summer. Took him to the vet for a regular checkup today and the vet said just a little water on a toothbrush should take care of it. He then proceeded to brush my dragons teeth (SERIOUSLY???) in the office and Tashi LOVED it! Oh brother! He said you don't even need them to open their mouth, but just gently brush back and forth as you see in the picture below. About 20 seconds of that and his teeth and gums were sparkly clean!! So silly!
Here are pictures of his black teeth from the summer.
And here is my little stubborn and SO spoiled little patient this morning!!