Sea slugs,Or NUDIBRACHS,are normally too small to cause any problems when they die.And they do die. I havn't heard of one living more than 6months in a home tank.I had some,and they all died.
What do the nudibranches in your tank look like?I would watch them most nudibranches eat corals or sponges. As for them being poisonous the phyllidia varicosa is very poisonous and will kill everything in a aquarium if it secrets its poison. There are nudibranches that you can keep in a aquarium, but they require a lot of work. Elysia crispata (lettuce nudibranches) eat green hair alge and are photosynthetic like clams. They will consume a lot of alge and you should make sure they have a good food supply. You might even have to grow it in a seperate aquarium. You can even find nudibranches that eat aiptasia anemones.
reefkeeper is right, I just read an article in a magazine on them, and they are hard to keep, and very hard to feed. Which is probably why they don't live long.