It's called Stress coat by API. It uses aloe vera to replace/fortify the slime coat on a fish. I'm not sure if it will help his antennae heal, but it can't hurt. Just keep the water clean and if his antennae were damaged, they should begin growing back.
Hermit crab shells are typically fish safe. As long as they are cleaned and they aren't the painted ones. (The paint tends to chip off so I can only imagine what it would do in water...)
happy owner of a wild type GFP axolotl named Percival and too many guppies to count.