If seahorses are to be kept with fish at all, they need to be kept with docile tankmates, such as (most, not all) gobies and blennies. Even clownfish are iffy, because they can be aggressive when eating, and ANY fish in the tank with a seahorse will compete with it for food.
It's best to keep them in a species-only tank.
Also, as was already mentioned, stinging corals cannot be kept with seahorses. Seahorses hitch on everything and can be badly hurt if they hitch on a stinging coral or anemone.
I hope you are able to move her/him to her/his own tank and if not, I hope you are able to find a good home for her/him.