Congealed Green Slime is a block that can be crafted, grown, or found. They can be harvested by hand, and no tool harvests them faster than any other, barring Efficiency upgrades.
When falling or jumping on to a Congealed Green Slime, you will bounce randomly (occasionally not at all, especially when falling extreme distances) though never higher than you fell/jumped from. You never take fall damage from falling on a Congealed Green Slime block, regardless of the distance you drop on to it from.
Standing on a Congealed Green Slime block confers the Jump Boost 3 buff for as long as you are standing on it, until you either walk off or jump.
Congealed Green Slime blocks can be crafted in a 2x2 grid using 4 Slimeballs, as shown below. They can also be uncrafted into yield their ingredients.
From Slimy Trees
You can find a number of these blocks generated as the trunks of the Slimy Trees that grow on Slime Islands. You can also grow new Slimy Trees from the Slimy Saplings those drops to farm additional Congealed Green Slime blocks