White is Good
When a canker sore
or "aphthous ulcer" is in the process of healing it turns a whitish color. You are on the mend.
Leave it alone, don't pop it! maybe some salt water or baking soda gargles...If you want to feel better but actually, nothing is shown to significantly speed up healing once it gets to this stage. You're in the home stretch....1-2 weeks more and you'll be able to suck lemons again.