well some species of slugs, nudibranchs, cucumbers, sea apples, and box/cowfish can all become toxic when they die in a aquarium
most nudi's are specialized feeders, which means they eat only one things, weather its a type of coral, algae, sponge etc you would have to consult a expert, a good book, or my personal favorite seaslugfourm.com
and as for the aptasia, i also recomend the peppermint shrimps, buy quite a few and if you dont want to keep them permanatly your going to be tearing down your tank anyways, then you can find them a home in the club or just trade them in to your fav LFS.