I think I'd see if I could find vet. If I couldn't, (KEEP IN MIND THAT I HAVE NO EXPERTISE IN THIS AREA) I'd probably dab a little betadine on it once or twice a day, rinse it off, dry it, and dab a little mycitracin on it. Not much, though, as I wouldn't want much medicine coming off in the water. That's what I would do if it were my newt, but I'm NOT saying that that's what you SHOULD do. I think I recall people treating seahorse injuries in this manner.