Humm I was told that the mucus layer was normal with loaches but now you've mentioned it... I can see she does feel/look different to the others. This past week they have all been under a huge amount of stress due to the tank issue. I'm a bit unsure come Monday when new tank comes on whst is...
I have a tank with mostly weather loaches in, I have 2 that are about 10" long, 5 that are about maybe 5" long, along with a bristle nose catfish, 3" rainbow shark and a 23 year old clown loach who wasn't meant to stay when I got him and his tank mates... about 3 years ago! Somehow he never left...