I've never heard of an axolotl getting velvet, and I personally don't think they can. Are you keeping your axolotl with fish? Axolotls shouldn't be mixed with any other species - I'd separate them right away and keep an eye on your lotl for any signs of stress or sickness. I really don't think axolotls can get velvet, but it doesn't mean it can't harm them. Good luck!