Not to worry
This is usually just some dead tissue getting ready to peel off.
May also be some buildup of debris. You might gently take a wet q-tip and see if it wipes off. If not, then just leave it. By today you should be gently rinsing with warm salt water 3 - 5 times/day. You should be leaving your denture out at night, as well. Any sore spots and you should see your dentist for adjustments. Good luck