My dogs a 5 year old dachshund and had many problems in the past. Hopefully this one is not as serious!?

Just a few days ago we noticed he was favoring the left side of his mouth when he eats. So when we looked into his mouth we seen an indent sort of thing that looks to me like a canker sore or something where he might have done when he was chewing on a denta bone because we noticed he was having trouble eating it. So we have been rinsing it with warm salt water to see if that would help and it is now turning white. Does anyone know what this could be?