Slime Islands are naturally generated chunks of land floating in the sky in the Overworld or in the lava ocean in the Nether. The slime in the nether is called Blood Slime.They are made of slime dirt and slimy grass. They also may sometimes have slime trees, which are made of Congealed Slime for logs and slimeball-dropping leaves. They usually have Slime Water ponds with congealed slime shores, which spawn slimes in the pool. Sometimes it can also spawn a King Slime.
Slimy grass blocks will spread to slimedirt and regular dirt. However, slimy grass that spreads to dirt will drop regular dirt when broken. Slimy grass drops slimedirt otherwise. Silk touch allows slimy grass blocks to be harvested.