I wonder if this is an April Fool story that is four months early (or eight months late)?
I didn't know that Weymouth had leafy sea dragons, and as I'm fond of syngnathids, I'll have to plan a visit.
It is indeed poisonous, but the dragons are fighting it not eating it. I suspect that the toxins are not water soluble.
Or...the Aquarium has made a terrible terrible mistake.