The the picture doesn't appear in the post, but link works if you right click on it and open it in a new tab.
As Samantha has mentioned, what are your water parameters, ammonia, nitrite, nitrate, temperature. Please could you post the results of your water test.
I'm not very experienced as a keeper, but I have seen something similar on another link a few months ago. It is possible for some axolotls to be allergic to things like aloe vera, which is sometimes found in some dechlorinators.
Have you changed anything recently, such as your brand of dechlorinator? Have you changed anything in your tank? Added any decor?
Sorry about all of the questions, just trying to work out what could be causing the problem.
Great news that Boba is still behaving normally though, as this would indicate that it is not causing major problems. :happy: