Thank you to those who have responded, here is an update on my little guy. I have not noticed him making the clicking noise for 4 or more days. I have double and triple checked my husbandry and everything seems to be okay as I detailed in my last reply. I also cleaned his entire cage, giving him completely new substrate. He just ate a fuzzy so I guess my intuition was right about needing the smaller size mouse. From what I could see of his mouth while feeding, the inside was pink as normal and free of any mucus of irritation. As far as I could tell his mouth looked the same as it has always looked. So I am going to hesitantly say that he is okay, thank goodness, and that I won’t be taking him to the vet any time soon. If anyone has more feedback that would be appreciated, but I am going to try to stop worrying as nothing seems out of place anymore.