[Delphinus]

Sorry to hear about your loss.

Ditto on the water change advice. Maybe you also want to run some carbon and filter floss. It could theoretically release additional things such as shed nematocysts, but as had been mentioned it won't be anywhere near like that of a deceased sea apple. Unfortunately, whatever it was capable of doing to your water, it has already done so: so some preventative maintenance in is order, just to be on the safe side. In most likelihood, the worst is probably already past.. so I'd not give it a huge worry other than the water change and the carbon.