It seems that lime jelly might have been one of the last things the people in the Secret Annex ate (or intended to eat) before the place was raided.

Therefore, I decided to look up a recipe. Voila!

"Lime Jelly" - My Stained Apron

https://mystainedapron.com/lime-jelly/

The lime jelly detail was mentioned in this book: 

"The Last Secrets of the Secret Annex..."

https://www.goodreads.com/book/show/62918585-the-last-secrets-of-anne-frank

It was probably made with pectin.