Your sea hare is both male and female, although I'm pretty sure it can not self-fertilize. Some slugs can carry sperm and lay fertile eggs at a later time; not knowing what species of sea hare you have, I can't even venture to say whether your hare's eggs could possibly be fertile or not. Many people eat sea hare eggs, so they can't be that gross.
Someone on here had a hare who laid eggs for months before dying (I assume it eventually died), so it isn't a given that your hare will die soon, but often in nature hares are seen in large breeding groups and then there is large die off. It is believed that many of the animals have a life span of about a year, but perhaps the trip to the breeding groups is stressful, or many go without feeding during breeding (or there is a lack of food.) You will just have to wait and see. Your hare will do no more damage in your tank when it dies than any other animal of the same size -- it is not toxic.
