Any changes to the setup that may have effected ph? Maybe adding more dw? Or how recently did you acquire it. He looks fat so surely not much to worry about, I've experience some fishes fins deteriorate a little when added to my tank, I've kind of got a lot of dw for my size tank which I know effects the ph. My fish usually grow them back within 2 weeks and completely adjust. However I have had fish I just simply couldn't keep for to long in my 120. I had an Asian USD and 4 line cat in a 30B just fine, once they were big enough to not be eaten I put them in the bigger tank and for whatever reason their skin deteriorated, rapidly over a few days, including their barbels and fins, put them back in the thirty, 4 line didn't make it but USD did, healed completely and was brought to an lfs at about 6-7". In he 120 I have some other cats just fine. My trachy did seem to shed a whisker and slime coat when first put in.
So it could just be acclimating. I wouldn't worry to much yet, especially if it's one fin.