Erm...if I'm not wrong if humans were stung by jellyfish they would apply vinegar.
Not sure if that could apply to the fish...maybe if u want to try can take it out of water and give the wound a quick swab of vinegar...then put the fish back into the take asap.. I'm not responsible for whatever might be the outcome..this is just an idea.
You should add some medication...reef-safe ones to improve the stress coat...can try feeding garlic..will boost its immune system to resist subsequent infection of the wound.