You must have NEON LIGHTS and CLIMBING VINE to make JELLYFISH. You can find this item in hair category. It takes 1d 05h 13m 53s to grow JELLYFISH from seed to tree. Breaking this block will give you 10 experience points. JELLYFISH can be broken. Seed mathematics is very complicated, but you should get at least 1 seed back from splicing JELLYFISH. Profiting with this might be questionable. Will you get enough blocks to build something? You just have to find out yourself.